🚀 go-pugleaf

    Date: Saturday, 2 April 1988  09:17-EST
    From: Walter Roberson <WCSWR%CARLETON.BITNET@MITVMA.MIT.EDU>

    Someone has recently asked me where the 'rule' governing the
    interpretation of '!' and '@', both in the same address, may be
    found. If you have the address <aaa!bbb@ccc.uucp>, should
    this be interpreted as aaa!bbb!ccc or ccc!aaa!bbb ? As far as I
    know, the latter is correct -- but that is simply my experience,
    and not something I've seen documented. Could someone please
    point me to the appropriate RFC or other document?

You lose.  There's no RFC because the community that uses RFCs as
their standards doesn't (formally) recognize "!" syntax.

In other words, the precedence is determined by whoever wrote the mail
code that you are using, and you can't count on any particular
combination of "!" and "@" being universally acceptable.

Now you know why even the usenet is moving away from use of "!" where
possible (q.v. the "smail" mailer program).

--Rob