Mail addresses are based on the ARPANET protocol listed at the end of this manual page. These addresses are in the general format user@domain where a domain is a hierarchical, dot-separated list of subdomains. For example, the address firstname.lastname@example.org 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 alternate route if that were more convenient or efficient. For example, at Berkeley, the associated message would probably go di rectly to monet over the Ethernet rather than going via the Berkeley ARPANET gateway. Abbreviation 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@monet'' without adding the ``berkeley.edu'' since it is the same on both sending and receiving hosts. Certain other abbreviations may be permitted as special cases. For exam ple, at Berkeley, ARPANET hosts may be referenced without adding the ``berkeley.edu'' as long as their names do not conflict with a local host name. Compatibility 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(1) command. 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 (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. Route-addrs. 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: <@hosta,@hostb:user@hostc> 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@domain'' part of the address to determine the actual sender. Postmaster 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. 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 RFC822 group syntax (``group:user1,user2,user3;'') is not supported except in the special case of ``LI group:;'' because of a conflict with old berknet-style addresses. Route-Address syntax is grotty. UUCP- and ARPANET-style addresses do not coexist politely.
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