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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

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

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

       could be encoded in a To field as

          To: God@heaven.af.mil,
            <@brl.mil:"a\"quote"@heaven.af.mil>,
              "The Almighty".One@heaven.af.mil

       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 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.

SEE ALSO

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

                                                                  addresses(5)