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
             @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

             [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
          interpreted 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-@[]
          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
          [email protected] to [email protected].

          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

             [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
          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

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

          could be encoded in a To field as

             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
          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)