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 Internet gateway.
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 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
[Note: The route-addr syntax is officially deprecated in RFC 1123 and should not
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
“postmaster” to which problems with the mail system may be
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
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
Internet Message Format, RFC
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.