Question about bangs (exclamation marks) in local-parts of addresses:
My reading of the expansion of local-part to dot-atom to atext is
that a bang is (1) permitted in the local-part of the address and (2)
may appear _anywhere_ in the local-part.
Specifically, constructions such as <!bob(_at_)domain(_dot_)com> and
<bob!(_at_)domain(_dot_)com> are permitted, even though they technically
parsing rules for bang addresses.
Internet mail standards don't specify the interpretation of local-parts;
such interpretation is done only by the domain named on the right side
of the @ sign. (for instance, nothing requires that a domain interpret
a local-part containing ! as if it were a uucp address)
So yes, the ! can appear anywhere in a local-part, but that doesn't mean
that the domain on the right side of the @ sign will consider that local
part a valid address.
!!foo!!!(_at_)example(_dot_)com is perfectly valid if example.com thinks it is.