MAILADDR(7) BSD Miscellaneous Information Manual MAILADDR(7)NAMEmailaddr — 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 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
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.
Certain old address formats are converted to the new format to provide
compatibility with the previous mail system. In particular,
is converted to
is converted to
This is normally converted back to the ``host!user'' form before being
sent on for compatibility with older UUCP hosts.
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.
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:
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 gener‐
ally 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
[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:
is routed as indicated in the previous example.
Every site is required to have a user or user alias designated ``postmas‐
ter'' to which problems with the mail system may be addressed.
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''
SEE ALSOmail(1), sendmail(8)
Crocker, D. H., Standard for the Format of Arpa Internet Text Messages,
Mailaddr appeared in 4.2BSD.
The RFC822 group syntax (``group:user1,user2,user3;'') is not supported
except in the special case of ``group:;'' because of a conflict with old
Route-Address syntax is grotty.
UUCP- and Internet-style addresses do not coexist politely.
BSD June 16, 1993 BSD