addresses - formats for Internet mail addresses
A mail address is a string of characters containing @.
Every mail address has a local part and a domain part. The
domain part is everything after the final @. The local part
is everything before.
For example, the mail addresses
all have domain part heaven.af.mil. The local parts are
God, empty, and @at@.
Some domains have owners. It is up to the owner of
heaven.af.mil to say how mail messages will be delivered to
addresses with domain part heaven.af.mil.
The domain part of an address is interpreted without regard
to case, so
all refer to the same domain.
There is one exceptional address that does not contain an @:
namely, the empty string. The empty string cannot be used
as a recipient address. It can be used as a sender address
so that the real sender doesn't receive bounces.
The qmail system allows several further types of addresses
in mail envelopes.
First, an envelope recipient address without an @ is
interpreted as being at envnoathost. For example, if
envnoathost is heaven.af.mil, the address God will be
rewritten as God@heaven.af.mil.
Second, the address #@ is used as an envelope sender
address for double bounces.
Third, envelope sender addresses of the form pre@host-@
are used to support variable envelope return paths (VERPs).
qmail-send will rewrite pre@host-@ as prerecip=domain@host
for deliveries to recip@domain. Bounces directly from
qmail-send will come back to pre@host.
CHOOSING MAIL ADDRESSES
Here are some suggestions on choosing mail addresses for the
Do not use non-ASCII characters. Under RFC 822 and RFC 821,
these characters cannot be used in mail headers or in SMTP
commands. In practice, they are regularly corrupted.
Do not use ASCII control characters. NUL is regularly
corrupted. CR and LF cannot be used in some combinations
and are corrupted in all. None of these characters are
usable on business cards.
Avoid spaces and the characters
These all require quoting in mail headers and in SMTP. Many
existing mail programs do not handle quoting properly.
Do not use @ in a local part. @ requires quoting in mail
headers and in SMTP. Many programs incorrectly look for the
first @, rather than the last @, to find the domain part of
In a local part, do not use two consecutive dots, a dot at
the beginning, or a dot at the end. Any of these would
require quoting in mail headers.
Do not use an empty local part; it cannot appear in SMTP
Avoid local parts longer than 64 characters.
Be wary of uppercase letters in local parts. Some mail
programs (and users!) will incorrectly convert
God@heaven.af.mil to firstname.lastname@example.org.
Be wary of the following characters:
Some users will not know how to feed these characters safely
to their mail programs.
In domain names, stick to letters, digits, dash, and dot.
One popular DNS resolver has, under the banner of security,
recently begun destroying domain names that contain certain
other characters, including underscore. Exception: A
dotted-decimal IP address in brackets, such as [127.0.0.1],
identifies a domain owned by whoever owns the host at that
IP address, and can be used safely.
In a domain name, do not use two consecutive dots, a dot at
the beginning, or a dot at the end. This means that, when a
domain name is broken down into components separated by
dots, there are no empty components.
Always use at least one dot in a domain name. If you own
the mil domain, don't bother using the address root@mil;
most users will be unable to send messages to that address.
Same for the root domain.
Avoid domain names longer than 64 characters.
ENCODED ADDRESSES IN SMTP COMMANDS
RFC 821 defines an encoding of mail addresses in SMTP. For
example, the addresses
could be encoded in RCPT commands as
RCPT TO:<The\ Almighty.One@heaven.af.mil>
There are several restrictions in RFC 821 on the mail
addresses that can be used over SMTP. Non-ASCII characters
are prohibited. The local part must not be empty. The
domain part must be a sequence of elements separated by
dots, where each element is either a component, a sequence
of digits preceded by #, or a dotted-decimal IP address
surrounded by brackets. The only allowable characters in
components are letters, digits, and dashes. Every component
must (believe it or not) have at least three characters; the
first character must be a letter; the last character must
not be a hyphen.
ENCODED ADDRESSES IN MAIL HEADERS
RFC 822 defines an encoding of mail addresses in certain
header fields in a mail message. For example, the addresses
could be encoded in a To field as
To: < "God"@heaven .af.mil>,
"a\"quote" (Who?) @ heaven . af. mil
, God<"The Almighty.One"@heaven.af.mil>
There are several restrictions on the mail addresses that
can be used in these header fields. Non-ASCII characters
are prohibited. The domain part must be a sequence of
elements separated by dots, where each element either (1)
begins with [ and ends with ] or (2) is a nonempty string of
printable ASCII characters not including any of
and not including space.
envelopes(5), qmail-header(5), qmail-inject(8), qmail-
Man(1) output converted with