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 dou-
       ble bounces.

       Third, envelope sender addresses	of the form pre@host-@[] are  used  to
       support	variable  envelope  return paths (VERPs).  qmail-send will re-
       write  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 @ 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	begin-
       ning,  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 destroy-
       ing  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	begin-
       ning, or	a dot at the end.  This	means that, when a domain name is bro-
       ken down	into components	separated by dots, there are no	 empty	compo-
       nents.

       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

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)



				       5		   s/qmail:(addresses)