.\"	$OpenBSD: mailaddr.7,v 1.10 2007/05/31 19:19:58 jmc Exp $
.\"	$NetBSD: mailaddr.7,v 1.3 1994/11/30 19:07:17 jtc Exp $
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.\"     @(#)mailaddr.7	8.1 (Berkeley) 6/16/93
.\"
.Dd $Mdocdate: May 31 2007 $
.Dt MAILADDR 7
.Os
.Sh NAME
.Nm mailaddr
.Nd mail addressing description
.Sh DESCRIPTION
Mail addresses are based on the Internet protocol listed at the end of this
manual page.
These addresses are in the general format
.Pp
.Dl user@domain
.Pp
where a domain is a hierarchical dot separated list of subdomains.
For example, a valid address is:
.Pp
.Dl eric@CS.Berkeley.EDU
.Pp
Unlike some other forms of addressing, domains do not imply any routing.
Thus, although this address is specified as an Internet address, it might
travel by an alternate route if that were more convenient or efficient.
For example, at Berkeley, the associated message would probably go directly
to CS over the Ethernet rather than going via the Berkeley Internet
gateway.
.Ss Abbreviation
Under certain circumstances it may not be necessary to type the entire
domain name.
In general, anything following the first dot may be omitted
if it is the same as the domain from which you are sending the message.
For example, a user on
.Dq calder.berkeley.edu
could send to
.Dq eric@CS
without adding the
.Dq berkeley.edu
since it is the same on both sending
and receiving hosts.
.Ss Compatibility
Certain old address formats are converted to the new format to provide
compatibility with the previous mail system.
In particular,
.Pp
.Dl user@host
.Pp
and
.Dl user@host.domain
.Pp
are allowed;
.Pp
.Dl host.domain!user
.Pp
is converted to
.Pp
.Dl user@host.domain
.Pp
and
.Pp
.Dl host!user
.Pp
is converted to
.Pp
.Dl user@host.UUCP
.Pp
This is normally converted back to the
.Dq host!user
form before being sent
on for compatibility with older UUCP hosts.
.Ss Case distinctions
Domain names (i.e., anything after the
.Dq @
sign) may be given in any mixture
of upper and lower case with the exception of UUCP hostnames.
Most hosts
accept any combination of case in user names, with the notable exception of
MULTICS sites.
.Ss Route-addrs
Under some circumstances it may be necessary to route a message through
several hosts to get it to the final destination.
Normally this routing
is done automatically, but sometimes it is desirable to route the message
manually.
Addresses which show these relays are termed
.Dq route-addrs .
These use the syntax:
.Pp
.Dl <@hosta,@hostb:user@hostc>
.Pp
This specifies that the message should be sent to
.Dq hosta ,
from there to
.Dq hostb ,
and finally to
.Dq hostc .
This path is forced even if there is a more efficient
path to
.Dq hostc .
.Pp
Route-addrs occur frequently on return addresses, since these are generally
augmented by the software at each host.
It is generally possible to ignore all but the
.Dq user@hostc
part of the address to determine the actual sender.
.Pp
[Note: The route-addr syntax is officially deprecated
in RFC 1123 and should not be used.]
.Pp
Many sites also support the
.Dq percent hack
for simplistic routing:
.Pp
.Dl user%hostc%hostb@hosta
.Pp
is routed as indicated in the previous example.
.Ss Postmaster
Every site is required to have a user or user alias designated
.Dq postmaster
to which problems with the mail system may be addressed.
.Ss Other networks
Some other networks can be reached by giving the name of the network as the
last component of the domain.
.Em This is not a standard feature
and may
not be supported at all sites.
For example, messages to CSNET or BITNET sites can often be sent to
.Dq user@host.CSNET
or
.Dq user@host.BITNET ,
respectively.
.Sh SEE ALSO
.Xr mail 1 ,
.Xr sendmail 8
.Pp
Crocker, D. H.,
.Em "Standard for the Format of Arpa Internet Text Messages" ,
RFC 822.
.Sh HISTORY
.Nm
appeared in 4.2 BSD.
.Sh BUGS
The RFC 822 group syntax
.Pq Dq group:user1,user2,user3;
is not supported
except in the special case of
.Dq group:;
because of a conflict with old
berknet-style addresses.
.Pp
Route-Address syntax is grotty.
.Pp
UUCP- and Internet-style addresses do not coexist politely.