MAILADDR(7)
NAME
mailaddr − mail addressing description
DESCRIPTION
These addreses are based on the ARPANET protocol, and have the general format: user@domain where a domain is a hierarchical, dot-separated list of subdomains. For example, the address eric@monet.Berkeley.ARPA is normally interpreted from right to left: the message above would go to the ARPA name tables (which do not correspond exactly to the physical ARPANET), then to the gateway, after which it would be sent on 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 was 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.
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.ARPA” could send to “eric@monet” without adding the “.Berkeley.ARPA” since it is the same on both sending and receiving hosts.
Certain other abbreviations may be permitted as special cases. For example, at many sites, ARPANET hosts can be referenced without adding the “.ARPA” as long as their names do not conflict with a local host name.
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 mixture of case in user names, with the notable exception of MULTICS sites.
Differences with ARPA Protocols
Although the UNIX addressing scheme is based on the ARPA mail addressing protocols, there are some significant differences.
At the time of this writing the only “top level” domain defined by ARPA is the “.ARPA” domain itself. This is further restricted to having only one level of host specifier. That is, the only addresses that ARPA accepts must be in the format “user@host.ARPA” (where “host” is one word). In particular, addresses such as: eric@monet.Berkeley.ARPA are not currently legal under the ARPA protocols. For this reason, these addresses are converted to a different format on output to the ARPANET, typically: eric%monet@Berkeley.ARPA
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. An address that shows 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 generally augmented by the software at each host. It is generally possible to ignore all but the “user@host” 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.