The bang is legal in the left-hand side (mailbox) of an Internet email
address. From the perspective of Internet standards, the mailbox string is
uninterpreted.
From the standpoint of an MTA's implementation, precedence between bang at
at-sign depend upon the needs of the organization that is running the
MTA. There are times when either precedence makes sense.
Back when it mattered, I argued that the address needed to be evaluated
using the rules of the network from which the message arrived -
so if a message arrived from SMTP, the @ sign should have precedence;
if it arrived from UUCP, the ! should have precedence. I'd still
argue that these are the right rules for the rare case where it matters.
Fortunately, these days we can generally avoid embedding routes in
addresses, so it doesn't matter very often anymore.
Keith