MAILADDR(7) BSD Miscellaneous Information Manual MAILADDR(7)
mailaddr -- mail addressing description
Mail addresses are based on the Internet protocol listed at the end of
this manual page. These addresses are in the general format
where a domain is a hierarchical dot separated list of subdomains. For
example, a valid address is:
Unlike some other (now obsolete) forms of addressing, domains do not
imply any routing, or the existence of a particular host. Simply because
mail may be sent to ``userATsomedomain.com'' does not imply that there is
any actual host named ``somedomain.com'', and does not imply a particular
routing of the message. Routing is performed by Mail Transport Agents,
such as sendmail(8), based on policies set in the MTA's configuration.
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. Whether abbreviation is permitted depends on how
your site is configured.
Domain names (i.e., anything after the ``@'' sign) may be given in any
mixture of upper and lower case. Most hosts accept any combination of
case in user names, although there are exceptions.
Every site is required to have a user or user alias designated ``postmas-
ter'' to which problems with the mail system may be addressed, for exam-
Certain old address formats, such as UUCP ``bang path'' addresses,
explicitly routed internet addresses (so-called ``route-addrs'' and the
``percent hack'') and others have been used historically. All these
addressing formats are now considered obsolete, and should no longer be
To some extent, sendmail(8) (when running with normal configuration
files) attempts to provide backward compatibility for these addressing
forms, but in practice many of them no longer work. Users should always
use standard Internet style addresses.
D. H. Crocker, Standard for the Format of Arpa Internet Text Messages,
RFC, 822, August 1982.
mailaddr appeared in 4.2BSD.
The RFC 822 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, not that anyone cares about either berknet or
group syntax style addresses any longer.
BSD June 16, 1998 BSD