mailaddr -- mail addressing description


Mail addresses are based on the ARPANET protocol listed in RFC 822. These addresses are in the general format:


where domain is a hierarchical, dot-separated list of subdomains. For example, the address is normally interpreted from right to left: the message should go to the ARPA name tables (which do not correspond exactly to the physical ARPANET), then to the Berkeley gateway, after which it should go to the local host monet. When the message reaches monet, it is delivered to the user eric.

Unlike some other forms of addressing, this does not imply any routing. Thus, although this address is specified as an ARPA address, it might travel by an alternative route if that were more convenient or efficient. For example, at Berkeley, the associated message would probably go directly to monet over the Ethernet rather than going via the Berkeley ARPANET 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 could send to eric@monet without adding the ``'' since it is the same on both sending and receiving hosts.

Certain other abbreviations may be permitted as special cases. For example, at Berkeley, ARPANET hosts may be referenced without adding the ``'' as long as their names do not conflict with a local hostname.


Certain old address formats are converted to the new format to provide compatibility with the previous mail system. In particular: user@host.ARPA is allowed, and host:user is converted to user@host in order to be consistent with the rcp(TC) command.

Also, the syntax 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.

The current implementation is not able to route messages automatically through the UUCP network. Until that time, you must explicitly tell the mail system which hosts to send your message through to get to your final destination.

Case distinctions

Domain names (that is, anything after the ``@'') 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 usually augmented by the software at each host. It is generally possible to ignore all but the user@domain part of the address to determine the actual sender.


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.

NOTE: 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.


The RFC 822 group syntax


is not supported except in the special case of


because of a conflict with old berknet-style addresses.

UUCP and ARPANET-style addresses do not coexist politely.

See also

mail(C), sendmail(ADMN)

RFC 822

© 2003 Caldera International, Inc. All rights reserved.
SCO OpenServer Release 5.0.7 -- 11 February 2003