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     Mail addresses are based on the ARPANET 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, 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 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.


     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


     Certain old address formats are converted to the new format to provide
     compatibility with the previous mail system.  In particular,


     is allowed and


     is converted to


     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

   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.


     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@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.  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'',


     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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