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The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Key Applications and Application Protocols            9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)                 9  TCP/IP Electronic Mail System: Concepts and Protocols (RFC 822, MIME, SMTP, POP3, IMAP)                      9  TCP/IP Electronic Mail Addresses and Addressing

TCP/IP Electronic Mail Addressing and Address Resolution 1 2 TCP/IP Electronic Mail Aliases / Address Books, Multiple Recipient Addressing and Electronic Mailing Lists

UUCP-Style Addressing

An older address style commonly associated with e-mail was the “UUCP style” address. The Unix-to-Unix Copy Protocol (UUCP) was commonly used years ago to route mail before SMTP became widely deployed (again, it is still used, just not as much as before). The addresses in this system are specified as a path of hosts separated by exclamation marks (“!”). The path dictates the route that mail takes to get to a particular user, passing through a series of intermediate machines running UUCP. For example, if mail to “joe” at the host “joesplace” had to go through three hosts “host1”, “host2” and “host3”, the address would be:

host1!host2!host3!joesplace!joe

Since the slang term for an exclamation mark is “bang”, this came to be called bang path notation.

The use of UUCP style notation was sometimes mixed with TCP/IP style domain name address notation when DNS came into use. So you might have seen something like “host1!user@domain”. There was some confusion in how exactly to interpret such an address: does it mean to send mail first to “host1” and then to “user@domain”? Or does it mean to first send it to the “domain” which then goes to “user” at “host1”? There was no universal answer to this. The problem was mostly resolved both by the decrease in use of UUCP and the move on the part of UUCP systems to TCP/IP style domain name addressing.

Finally, you may encounter e-mail addresses that appear like multiple TCP/IP addresses that have been nested using unusual punctuation. For example, you may see something that looks like this:

user%domain1.com@subdomain.domain2.edu

This is a way of addressing sometimes seen when e-mail gateways are used; it will cause the mail to be sent to “user%domain1.com” at “subdomain.domain2.edu”. The address then is interpreted as “user@domain1.com”. However, again, not all systems are guaranteed to interpret this the same way.

E-mail gatewaying is not a simple matter in general, and as you can see, one reason is the use of different e-mail address styles and the problems of consistency in how complex “hybrid” addresses are interpreted, as discussed above. However, as the Internet expands and TCP/IP becomes more widespread, it is becoming less and less common to see these older, special address formats in use. They are becoming more and more a “historical curiosity” (unless you happen to use one of them!)

TCP/IP Electronic Mail Addressing and Address Resolution 1 2 TCP/IP Electronic Mail Aliases / Address Books, Multiple Recipient Addressing and Electronic Mailing Lists