TCP/IP Historical and Special Electronic Mail Addressing
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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:
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:
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 firstname.lastname@example.org. 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!)
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