POP3
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... can be implemented by different application-layer protocols (e.g.,
POP3, IMAP4). A tag, such as "RetMail", could be registered for it.
...
... service. These are also associated with IANA-registered tags. Using
the mail example above, "POP3" and "IMAP4" could be registered as
application protocol ...
... protocol tags should be specific enough that finding a known pair
(e.g., "RetMail:POP3" would be sufficient for a client to identify a
server with which it can communicate.
...