Before email (short for electronic mail, or e-mail) when dinosaurs walked the Earth, it was difficult to avoid “phone tag”. To leave someone a message, you needed to physically travel to their office and leave a slip of paper. (There were no cell phones in those days, and even answering machines were rare.) At some point it was realized that (at some organizations anyway) people had computer terminals in their offices, and that it was possible to leave a message in a file at a given location. When a user returned to their office they could check if that file existed and contained any new messages.
To make this easier, a simple program was used.
To send email to someone, the original
Any text you typed thereafter would be appended to that user's
mailbox file, sometimes called the inbox.
(The standard location back then was
When done entering the message, you would indicate EOF
by hitting Control+D.
(You could also pipe into this command.)
To read your mail (if any), a user would just type
Sometimes the original message is quoted in the new message
body as well.
Email continues to be very popular. The marketing research firm Radicati reports over 182 billion email messages are sent every day (in 2013). That comes to over 2 million email messages sent or received every second! (Sadly, other estimates indicate that over 90% of that is spam or other forms of malware, such as viruses.)
The invention of email is generally credited to V.A. Shiva Ayyadurai in 1978, while he was a high school student in New Jersey. However, this isn't true at all; email was in use in the early 1970s. Apparently, this guy invented a program he called “EMAIL”, and the press got confused.
An email message contains three parts: the envelope which identifies the sender and recipients, the headers, and the message body. The headers and body together are referred to as the message.
It is important to understand that only the recipients listed on the envelope will receive an email message; mail servers never look at any headers to determine this! Also, the envelope is not part of the message that gets saved in a user's mailbox. So once some email message is delivered to you, you can't tell who else was listed on the envelope.
The special header mentioned above that identifies the start of an email message in a mailbox looks like this:
From sender date
(Note the space and no colon after
This special “
From ” header is part of the
mailbox format common to most systems, known as
MBOX or Berkeley mailbox format,
and is not a standard email header at all.
See RFC-4155 and
mbox(5) man page for details.)
header is generated automatically from the
envelope from address.
Even if a “
From ” header was provided, it is
over-written by most mail servers with the real sender.
This header is always the first one of an email message.
When a mail program is reading a mailbox, an email message begins
with this header and continues until the next
From ” header, or until the end of the file.
What happens if the message body contains a line starting with
The mail program will typically insert an ASCII space
in front of that line, a technique called
When displaying messages the mail program removes the extra
Eventually other headers were allowed as well such as
A problem is, how to tell the difference between the headers
added by the mail program and the message entered by the
The answer is to have all the headers at the beginning
(hence the name), followed by one blank line, and that
followed by the message entered by the user.
That part is known as the message body.
Recall the “
To:” header does not determine
to whom the email gets delivered.
The addresses passed to the mail server (the envelope
addresses) and not the ones listed in any mail headers
determine who receives the email.
Since the various headers in the message do not determine who
receives the message, they may be faked easily.
When composing email, the user is asked to set the recipients, using
All three will add the recipients to the envelope, but only the
To:” and “
recipients will be listed in any email headers.
Bcc:” recipients are added to
the envelope only; there is no such thing as a
There are many standard headers that can be used, such as:
Cc: (carbon copy),
The carbon copy list is the same as the “To:” list, just
more recipients to add to the envelope.
The only difference is that somehow being listed on the
To: list confers more status than being listed on the
Note there is no such thing as a
a “Blind carbon copy” recipient is listed on the
envelope but not in any headers.
for a description of standard email headers.
It is often the case where you need to add your name, title, contact information, and sometimes a legal notice, to some or to every email message you send. This information is known as a signature block, signature line, sig block, or just a signature. (This should not be confused with digital signatures, discussed below.) That can get tedious to type in for each message! Many mail programs (MUAs) and mail servers (MTAs) have a feature where you can set a signature to be automatically appended to the body of all email messages.
There are rules of “netiquette” (network etiquette) for email signatures. They should always begin with a line only containing two dashes and a space. This signature separator is called sig dashes, signature cut line, or sig-marker. (It is recognized automatically by most email programs, which can treat signatures specially when replying to a message and quoting the message body.) The other rules are that the signature should be plain text, with no more than 4 lines; each line should be at most 80 columns long.
There are many rules of “netiquette” for the body of email.
When using traditional, plain text email there are rules for
formatting signatures, quoting material, line length, line wrapping,
and so on.
These are defined in
RFC-3676 (Text/Plain Format).
This also defines when to use “space stuffing” (adding
a space to lines that start with “
a space, or a “
One useful convention you should follow is to quote URLs with angle-brackets (“<” and “>”). This allows an MUA to recognize a URL even when wrapped over multiple lines.
Modern email addresses look like this:
hostname is a host or computer on a
The “@hostname” part is optional;
if missing usually
(that is, the current system) is assumed.
The hostname should be a valid
host name, or an IP
address enclosed in square-brackets
It can be another name defined in the DNS system,
in an MX record.
A common example is to use an organizations domain name only
and not the name of any particular host; for example
firstname.lastname@example.org” and not
The username should be a valid account on that system
(or a defined alias such as
There are a number of programs play a role in composing, reading, and delivering email:
MTA (Mail Transport Agent) — Examples include Sendmail, Postfix, Exim, and Exchange. The MTA is the software that accepts email from an MUA (see below) and then routes and forwards the email (several hops if necessary) to the destination MTA. The destination MTA also must handles security issues such as rejecting email or sending a redirection message back, alias expansion, forwarding, relaying, etc. An MTA that accepts mail destined for other MTAs is relaying email. An MTA that does this without requiring sender authentication is called an open mail relay. Spammers love these!
(Mail Delivery Agent) —
An example is
The MDA handles final delivery issues such as virus
scanning, spam filtering, return-receipt handling, automatic mail
processing (by piping into some program), forwarding email
to users and groups, sorting email into different mailboxes,
The most common action is to simply append to user's inbox.
(Note some software such as Exchange or Sendmail includes both an
MTA and MDA, and possibly additional software.)
The MDA must be configured
with the location of a user's inbox.
The standard location is
but this can be changed (how depends on which MDA
Of course the MUA must also know that pathname, if it
accesses the mailbox directly.
(If not, the MAA must be configured with that pathname, and
the MUA must be configured to use the MAA.)
MAA (Mail Access Agent) — Examples include Courier and Cyrus. When email is sent to a user, the MDA stores it on the server's hard disks. Users rarely have login access to that mail server! (YborStudent is an exception.) Consider AOL mail, Yahoo mail, Gmail, or Hotmail. Your mailboxes are stored on those remote servers. Somehow you need to access those remote mailboxes with your local mail reading software (your MUA). An MAA server is used in addition to the MTA to provide this remote access. The user's MUA authenticates the user to the MAA which then downloads a copy of the user's mailbox (or selected messages only) to the MUA, where the user can then read, save, copy, print, or delete messages. (Remember on some systems, notably Exchange server, the MTA, MDA, and even MAA are a part of a single program.)
(Mail User Agent) —
Outlook, Thunderbird, HotMail.com,
The MUA (also called an email client)
is the software that allows you to compose,
send, and read your email.
An MUA must be configured with the MTA
to use to send mail and the MAA
to use to fetch the mail.
(Older MUAs such as
mailx can't use an
MAA; they just read the mailbox file directly.)
In addition some MTAs and MAAs require
usernames and passwords for authentication and may also require
additional security configuration.
Some MUAs such as
(or Gmail) are not installed on your local computer,
but on a web server that users access with a web browser.
Even though they are installed on a server they are still just
MUAs or email clients.
MSA (Mail Submission Agent) — With about 90% of email currently rejected as spam or as containing viruses, a commonly used mail architecture today is to use an MSA to screen out and drop such email before the MTA and MDA must process it. In this case the MDA won't also need to scan for viruses or spam (although it can, perhaps using a more sophisticated filter to catch spam that makes it through the bulk filter used in the MSA). MSAs can be accessed with a custom protocol (see below). More commonly, the MSA for an organization “pretends” to be the MTA, so the outside world doesn't know there is an MSA at all.
First you start up your MUA, compose a message, add
some headers (such as
Subject:), and state to whom
the email should be sent.
When you are finished (and click on “Send”),
the MUA (usually) adds some
additional standard headers and then sends the mail message to your
The mail gets routed from MTA to
MTA (nowadays very few hops are needed), with the
MTAs in the middle relaying the mail
to the next MTA along the path from the source to
(Each MTA that receives an email will add a
header to it.
By looking at these headers you can see the path a message took.)
The mail arrives and is accepted by the MTA at the
That MTA gives the mail to MDA which
may filter the mail, forward it, sort it to
different mail folders, ..., and finally deliver the mail to
the user's inbox.
Note when sending email to many users on the same system,
all the users are listed on
RCPT: line (envelope
addresses, not a header) but the actual email is
sent only once to that (remote) system.
The recipient is somehow notified of the arrived email,
and eventually reads it.
biff is a mail notification program named after a
(It's not true that Biff used to bark at the mailman,
that's just a myth.)
Use the arguments “
y” or “
to enable or disable these notifications.
biff is annoying but if you have a
xbiff is useful.
use the mail notification feature of the shell (most shells
have such a feature), which only notifies
you between running commands and not in the middle of some task
The MTAs work in a store and forward manner. An MTA receives an email message and stores it on a (local) disk. Then the message is relayed to another MTA or handed to the local MDA for delivery. Various problems can cause messages to not get delivered. This is usually known as a bounced message.
Mailer-daemon is the usual name of an MTA or MDA when it generates error email messages to return to the sender. Common causes include: bad user-name (destination MTA will bounce the email, which means return it to the sender with an explanation as to what happened), bad hostname (sender's MTA will bounce it), destination MTA is down (Sender's MTA—actually, the MTA immediately upstream of the destination—will try for awhile and then send a warning. If the destination server never responds then the MTA will eventually give up and will bounce the email).
The different components in the mail system must communicate with each other, passing the mail messages and other information. The rules of communication and the definition of message formats are called protocols. There are a number of standard protocols defined so that different vendor's software can interoperate easily (as well as the different components of an email system). The most important protocols are:
SMTP (Simple Mail Transport Protocol) — ESMTP is the modern enhanced version. (A good mnemonic for SMTP is “Send Mail To People”.) MUAs use this protocol to talk with MTAs; MTAs use it to talk to each other. Interestingly this protocol is designed to be used interactively by humans! ESMTP is defined by RFC-5321 and RFC-5322. (It was originally defined by RFC-821 and RFC-822, which was replaced with RFC-2821 and RFC-2822. It is still common to talk about “822” email.)
POP (Post Office Protocol) — Sometimes called POP3 (since POP is such a popular acronym adding the version number makes the name stand out), this protocol is used between an MUA and an MAA. POP is popular with ISPs because it is simple and cheap to implement. It allows you to send a username and password, then your entire mailbox is downloaded to your MUA. The only option is whether to delete the mailbox contents from the server after downloading, or not. POP is defined by RFC-1939.
POP3 assigns each mail message a unique ID
called the UIDL,
so it can tell which messages have been downloaded already.
In addition some vendor's have implemented an extension to
POP3 called “
that allows clients to transmit outbound mail.
(Normally SMTP is used for that.)
IMAP (Internet Message Access Protocol) — Like POP, IMAP is a protocol used between an MUA and an MAA. IMAP is more powerful and flexible than POP but takes more resources so ISPs rarely offer it. IMAP allows for selective message downloading and deleting, downloading of headers only, multiple mailboxes, and more. IMAP is defined by RFC-3501.
Some vendor's use proprietary protocols to talk between their proprietary mail servers and MUAs. Examples include Novell's “GroupWise”, IBM's “Lotus Notes”, and Microsoft's “Exchange”. Some companies have reverse-engineered these protocols and claim to have compatible products but it is not a good idea to rely on those. An “email gateway” is an MTA that translates between standard email protocols and some proprietary system's protocols.
One point to note about all these protocols is that the usernames and passwords are sent in plain text. Variations of all three (ESMTP, POPS and IMAPS) allow for encryption to protect usernames and passwords (and the contents of the email messages). However few ISPs support these protocols since they require more resources (and hence are more expensive to implement).
In the old days email was plain ASCII text, which takes only seven bits of each byte. Much of the early Internet dropped the 8th bit of every byte to gain a 12.5% speedup. Naturally this won't work with binary files such as GIFs, binary data files, or programs. So these needed to be encoded (in essence adding a zero bit after every seventh bit) when sent, and decoded by the recipient. Here's how this was done:
uuencode filename filename > file.uu mail recipient < file.uu
The encoded file is copied into the body of the outgoing email. Once delivered the recipient would have to save the body, edit it to remove all but the encoded file, and decode the file:
mail ... save received email body in “file.uu” ... uudecode file.uu
What a pain! Besides this problem, plain text email is... plain-looking. Early business adopters of email wanted better looking email with features such as bold, italics, underline, justified text, and color.
MIME (Multipurpose Internet Mail Extensions) is a protocol that MUAs use to provide styled text demanded by business users of early email. Today that isn't important (as we now use HTML for email with styles, graphics, and fancy formatting). But most importantly MIME supports multi-part email messages. This is when the body of the message is split into several parts, separated by a MIME separator string, where each part contains its own headers and is automatically encoded and decoded. Each of these parts are known as an attachment. Note that MIME is invisible to MTAs, MDAs, and MAAs, which only see a single message body with some weird stuff in it. (Virus scanners do know about attachments, of course.)
View a sample email message that uses MIME attachments.
Many old command line MUAs exist, the most popular of
mailx doesn't know about MAAs
An updated compatible MUA
available (nail = new mail?).
nail is often installed under the name
on Linux and some Unix systems.
These older MUAs are still valuable because some of
mailx but not
alpine) can be used
non-interactively to send mail from a shell script, and because
system administrators often use command line access to
Unix/Linux servers via
and may need to read or send mail directly from that server.
The use of a MUA such as
should be easy to learn since it is menu-driven.
is the current version of the
(The developers wanted to change the license before continuing
development, and they couldn't with the old name.)
alpine, at any point you can examine the menus
to see what you can do.
These are context-sensitive menus.
For instance hitting
^J (control+J) when
a header field is highlighted means to add an attachment;
if the message body is highlighted this means to
justify the text.
In message body use
^R to read a file
and paste its contents into the mail body.
Spam is usually sent from personal computers infected by a virus, making them part of a “botnet”. The Kelihos botnet controlled 41,000 computers worldwide, and was capable of sending 3.8 billion spam e-mails per day.
One type of nasty spam that is popular is a message that appears to be from someone or some organization you know. The attacker tries to trick you into clicking a link (in the email body) that will either run nasty software on your computer or take you to a fake website that looks real, to collect your personal information. This is known as phishing and can be very hard to detect. (Avoid clicking on links in email; use a browser's bookmark/favorite instead to be sure you're going to the correct site.)
In the U.S. and a few other countries it is legal for users to encrypt their emails to protect their privacy. (See below for details on how to do this.) However recent legislation (such as the “Sarbanes-Oxley Act”, or “SOX”) may require that for any organization that allows encrypted email, the organization must be able to recover the private key to be able to turn over emails when legally requested to do so by the proper authorities.
In other parts of the world it is illegal to encrypt email.
A related issue is censorship of emails by employers, ISPs, or governments. Today there is wide-spread censoring or delaying of some Internet traffic by many ISPs. Some ISPs have a “premium” level of service where they promise not to delay or block your Internet traffic if you pay extra. (To me this seems a kind of protection racket: “Nice email you got there buddy! If you pay I can make sure nothing bad will happen to it”.)
Return-Receipt-To: "Hymie Piffl" <email@example.com>
Or this official header (RFC 3798):
Disposition-Notification-To: J Smith <firstname.lastname@example.org>
For more details, see wikipedia.org/wiki/Return_receipt.
A list of e-mail addresses identified by a single name
email@example.com is called
a mailing list.
When an e-mail message is sent to the mailing list name
it gets sent to all the addresses in the list.
Each list also has a special email address for configuring
your use of the list (for example, add or remove yourself
from the list) and another address for the (human) owner of
A good resource for learning about mailing lists is
“Understanding Mailing Lists” by Harley Hahn
(the author of our textbook).
Users can configure the
to manage mail logs, to spam check, to filter email, to
automatically reply to some mail, etc.
See the man pages for
contains examples of MDA return-receipts and spam
filter using regular expressions,
While you have no guarantee of privacy with your email, you are allowed (in the U.S. anyway) to protect your email by encrypting the message. Such a message can't be read or tampered with by unauthorized parties.
The older technology for encryption is called symmetric (or shared) key: you and I share a key (password). Qu: how do we do that securely? Qu: what about doing business say with Amazon.com using this?
The old method is efficient but the problems are too difficult to make this technology useful on a wide scale. A newer technology is called public key encryption. With this method a pair of keys is made for each party. One is kept secret (the private key) and one is published (in email messages, in flyers, on web sites, on key servers, etc.) called the public key. To send a message to you I encrypt the message with your public key. Only you can decrypt it since this requires your private key and only you have it.
You reply to me by encrypting your reply with my public key (which only I can decrypt, using my private key). As you can see, four keys are used altogether. Public key encryption is the technology behind secure web sites that we all rely on (the web sites using the HTTPS protocol).
Public key encryption is much, much slower than symmetric key encryption. To make this technology practical, rather than encrypt a lengthy email message (body) a very large random number is generated by the sender. This number is used as a symmetric key and the message is encrypted with it. Only this session key gets encrypted using the public key method.
A digital signature is created for a message by encrypting it with the sender's private key. This doesn't protect the confidentiality of the message since anyone with the sender's public key can decode the message. (If privacy is also desired, the sender encrypts this encrypted message with the recipient's public key.) If the sender's public key decodes the message correctly, it is strong proof that their private key was used to encrypt it in the first place. Since only the sender has their private key, only the sender could have sent the message. Note this encryption with the sender's private key is a digital signature; a GIF graphic of a hand-written signature is not! (View sample digitally signed email.)
The U.S. federal government now treats digital signatures just as binding as a hand-written (or holographic) signature. See the Electronic Signatures in Global and National Commerce (ESIGN) Act passed in June of 2000, for details. Also many state governments have passed laws treating digitally signed emails as equivalent to documents signed holographically (by a person).
In practice this takes too long (even on modern computers) so a checksum (or digest or hash) of the message is encrypted with the private key instead, and this is appended to the (unencrypted) message. The recipient also computes a message digest of the email, then decrypts the message digest sent with the message body and compares the two. If the two digests differ the message was altered or forged.
Because the private keys used to encrypt email must often be made available to organizations (to comply with laws), separate sets of keys are often used for digital signatures and for email encryption.
Issue of Trust: A public key can be digitally signed by a trusted third party (such as VeriSign). This third party has a well-known public key. Most web browsers and email clients come with a built-in list of such well-known public keys.
People can use
(Pretty Good Privacy, Gnu Privacy Guard, both
written by Phil Zimmerman) to encrypt, decrypt, compute
message digests, and to digitally sign messages.
PGP was written first; later versions were
GPG is sometimes called
(See the man page for
gpg for details.)
GPG provides easy email integration with modern MUAs. But to use this technology people must generate a pair of keys and publish their public key. (You also need a third party to sign your key so others will trust it.) These complexities have hampered the widespread adoption of encryption and digital signatures.
The secure web sites with URLs such as
work by exchanging public keys between
server and browser, which then verifies these by using the
trusted third party's public key to validate the key.
Next the browser and web server exchange a session key
(a big random number encrypted with the public keys)
to use for symmetric key encryption for the rest of that session.
(This is an over-simplification of what really happens but
should provide some idea of the process.)
mailx. These tools are still useful to allow mail to be send from a shell script.
/var/mail/username. This can be changed by the MDA. The MUA must also know this location; the
From user date”.
Subject:, etc. (However there is no
Received:” header to the front of the email.
xbiff(or similar GUI program), or by the shell's email notification feature (which relies on
uudecodeutilities were common, now MIME's Base-64 encoding is used.
alpinehas an easy to use menu-driven interface.