Ubuntu Manpage: addresses - formats for Internet mail addresses

Provided by: qmail_1.06-6.2~deb10u1build0.20.04.1_amd64

NAME

       addresses - formats for Internet mail addresses

INTRODUCTION

       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

          God@heaven.af.mil
          @heaven.af.mil
          @at@@heaven.af.mil

       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

          God@heaven.af.mil
          God@HEAVEN.AF.MIL
          God@Heaven.AF.Mil

       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.

QMAIL EXTENSIONS

       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 Internet.

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 an address.

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 commands.

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 god@heaven.af.mil.

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

          God@heaven.af.mil
          a"quote@heaven.af.mil
          The Almighty.One@heaven.af.mil

       could be encoded in RCPT commands as

          RCPT TO:<God@heaven.af.mil>
          RCPT TO:<a\"quote@heaven.af.mil>
          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