MAILADDR(7)

NAME
mailaddr - mail addressing description

DESCRIPTION
Mail addresses are based on the Internet protocol listed at the end of this manual page. These addresses are in the general format

    user@domain

where a domain is a hierarchical dot separated list of subdomains. For example, a valid address is:

    eric@CS.Berkeley.EDU

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.

  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 “calder.berkeley.edu” could send to “eric@CS” without adding the “berkeley.edu” since it is the same on both sending and receiving hosts.

  Compatibility
Certain old address formats are converted to the new format to provide compatibility with the previous mail system. In particular,

    user@host

and
    user@host.domain

are allowed;

    host.domain!user

is converted to

    user@host.domain

and

    host!user

is converted to

    user@host.UUCP

This is normally converted back to the “host!user” form before being sent on for compatibility with older UUCP hosts.

  Case distinctions
Domain names (i.e., anything after the “@” 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.

  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 “route-addrs”. These use the syntax:

    <@hosta,@hostb:user@hostc>

This specifies that the message should be sent to “hosta”, from there to “hostb”, and finally to “hostc”. This path is forced even if there is a more efficient path to “hostc”.

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 “user@hostc” part of the address to determine the actual sender.

[Note: The route-addr syntax is officially deprecated in RFC 1123 and should not be used.]

Many sites also support the “percent hack” for simplistic routing:

    user%hostc%hostb@hosta

is routed as indicated in the previous example.

  Postmaster
Every site is required to have a user or user alias designated “postmaster” to which problems with the mail system may be addressed.

  Other networks
Some other networks can be reached by giving the name of the network as the last component of the domain. 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 “user@host.CSNET” or “user@host.BITNET”, respectively.

SEE ALSO
mail(1), sendmail(8), smtpd(8)

STANDARDS
P. Resnick, 2008, RFC 5322, Internet Message Format.

HISTORY
mailaddr appeared in 4.2BSD.

BUGS
The RFC 5322 group syntax (“group:user1,user2,user3;”) is not supported except in the special case of “group:;” because of a conflict with old berknet-style addresses.

Route-Address syntax is grotty.

UUCP- and Internet-style addresses do not coexist politely.