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  The local parts are
       God, empty, and @at@.

       Some domains have owners.   It  is  up  to  the  owner  of  to  say how mail messages will be delivered
       to addresses with domain part

       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,  the  address  God will be
       rewritten as

       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  the  owner  hack.   qmail-send  will
       rewrite  pre@host-@[] as prerecip=domain@host for deliver-
       ies to recip@domain.   Bounces  directly  from  qmail-send
       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 prop-

       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

       Avoid local parts longer than 64 characters.

       Be wary of uppercase letters in local  parts.   Some  mail
       programs    (and    users!)   will   incorrectly   convert to

       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  secu-
       rity,  recently begun destroying domain names that contain
       certain other characters,  including  underscore.   Excep-
       tion:  A  dotted-decimal  IP  address in brackets, such as
       [], identifies a domain owned by whoever owns the
       host at that IP address, and can be used safely.
       at the beginning, or a dot at the end.  This  means  that,
       when  a  domain  name is broken down into components sepa-
       rated 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.


       RFC  821  defines  an  encoding of mail addresses in SMTP.
       For example, the addresses


       could be encoded in RCPT commands as

          RCPT TO:<>
          RCPT TO:<a\">
          RCPT TO:<The\>

       There are several restrictions in  RFC  821  on  the  mail
       addresses  that  can be used over SMTP.  Non-ASCII charac-
       ters 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  char-
       acters  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.


       RFC 822 defines an encoding of mail addresses  in  certain
       header  fields  in  a  mail  message.   For  example,  the


       could be encoded in a To field as

              "The Almighty"

            "a\"quote" (Who?) @ heaven . af.  mil
            , God<"The Almighty.One">

       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-remote(8), qmail-smtpd(8)