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.
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.
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