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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, and are allowed;
is converted to and 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 sender.
[Note: the route-addr syntax is officially deprecated in RFC 1123 and should
not be used.] 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 addressed. Some other networks can be reached by
giving the name of the network as the last component of the domain. 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.
Crocker, D. H., RFC822.
appeared in 4.2 BSD.
The RFC822 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.
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