addresses(5)       Headers, Tables, and Macros       addresses(5)



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

        [email protected]
        @heaven.af.mil
        @[email protected]@heaven.af.mil

     all have domain part heaven.af.mil.   The  local  parts  are
     God, empty, and @[email protected].

     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

        [email protected]
        [email protected]
        [email protected]

     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  inter-
     preted as being at envnoathost.  For example, if envnoathost
     is heaven.af.mil, the  address  God  will  be  rewritten  as
     [email protected].

     Second, the address #@[]  is  used  as  an  envelope  sender
     address for double bounces.

     Third, envelope sender addresses of  the  form  pre@host[email protected][]
     are  used to support variable envelope return paths (VERPs).
     qmail-send will rewrite pre@host[email protected][] as prerecip=domain@host



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     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  cor-
     rupted.   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 pro-
     grams     (and     users!)    will    incorrectly    convert
     [email protected] to [email protected].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



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     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 [email protected];
     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

        [email protected]
        a"[email protected]
        The [email protected]

     could be encoded in RCPT commands as

        RCPT TO:<[email protected]>
        RCPT TO:<a\"[email protected]>
        RCPT TO:<The\ [email protected]>

     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 sur-
     rounded by brackets.  The only allowable characters in  com-
     ponents  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

        [email protected]
        a"[email protected]
        The [email protected]

     could be encoded in a To field as




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        To: [email protected],
          <@brl.mil:"a\"quote"@heaven.af.mil>,
            "The Almighty"[email protected]

     or perhaps

        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  ele-
     ments  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.

SEE ALSO
     envelopes(5),   qmail-header(5),   qmail-inject(8),   qmail-
     remote(8), qmail-smtpd(8)































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