mailaddr(7)mailaddr(7)Namemailaddr - mail addressing description
Mail addresses are based on the ARPANET protocol listed in the SEE ALSO
section of this reference page. In addition, the DECnet address format
can be used, if DECnet is installed on your system.
To send mail to DECnet users, use the following address format:
In this case, nodename is the name of the DECnet system, or host, on
which the target user resides.
Addresses based on the ARPANET protocol use the following general for‐
A domain is a hierarchical dot-separated list of subdomains. For exam‐
ple, the following address is interpreted from right to left:
In the previous example, the message is directed to the ARPA name
tables (which do not correspond exactly to the physical ARPANET). The
message then proceeds to the Berkeley gateway, after which it is
directed to the local host When the message reaches it is delivered to
Unlike some other forms of addressing, this does not imply any routing.
Thus, an address that is specified as an ARPA address can travel by an
alternate route if that route is more convenient or efficient. For
example, from Berkeley, the message could go directly to over the Eth‐
ernet, rather than using the Berkeley ARPANET gateway.
Abbreviations. In some instances, you do not have to type an entire
domain name. Generally, any information that follows the first dot may
be omitted if the sending and receiving domains are the same. For
example, a user on calder.Berkeley.ARPA can eliminate the .Berke‐
ley.ARPA when sending to eric@monet, because the sending and receiving
hosts are the same.
Other abbreviations are permitted when conflicts do not exist. For
example, Berkeley ARPANET hosts can be accessed without adding the
.ARPA, if their names do not conflict with a local host name.
Compatible Addresses. To provide compatibility with the previous mail
system, some old address formats are converted to the new format. In
particular, host::user is converted to user@host providing consistency
with the command.
The syntax host!user is converted to user@host.UUCP. Before being sent
on, user@host.UUCP is normally converted back to the host!user form.
This conversion is done for compatibility with older UUCP hosts.
The current implementation cannot automatically route messages through
the UUCP network. Thus, you must explicitly tell the mail system all
the hosts your messages must be sent through to arrive at its final
Case Distinctions. Domain names (that is, anything following the at
sign (@)) can be a combination of upper- and lowercase characters with
the exception of UUCP hostnames. Most hosts accept both upper- and
lowercase in user names, with the exception of MULTICS sites.
Differences with ARPA Protocols. Although the UNIX addressing scheme
is based on the ARPA mail addressing protocols, there are some signifi‐
Currently, the only top level domain defined by ARPA is the .ARPA
domain itself. This is further restricted to having only one level of
host specifier. That is, the addresses that ARPA accepts must be in
the format user@host.ARPA (where `host' is one word). For example, the
following address is not legal under the ARPA protocols:
Thus, the previous address would be converted to a different format on
output to the ARPANET. For example:
Route-addrs. In some instances, a message must be routed through sev‐
eral hosts to reach its final destination. Usually, this action is
invisible to the sender; however, it is sometimes desirable to route a
message manually. Addresses that are moved manually are called route-
addrs. The syntax is as follows:
The previous example directs the message to hosta, to hostb, and
finally to hostc. This route is used regardless of a more efficient
path to hostc.
Route-addrs occur frequently on return addresses, because they are gen‐
erally augmented by the software at each host. It is possible to
ignore all but the user@host part of the address to determine the
Postmaster. Every site must have a user or user alias designated as
postmaster to which problems with the mail system can be addressed.
CSNET. To send messages to CSNET, use the following syntax:
See Alsomail(1), sendmail(8)
Crocker, D. H., Standard for the Format of Arpa Internet Text Messages,