if To: has a given address and nothing else

Here's what I'm trying to figure out.

I want to match unless every address in To: matches a pattern.  That will
probably require a fork of formail -zx if there are more than one To: header,
and it will also require comment stripping or some bullet-proof comment ig-
noring.

The MTA will have rewritten the incoming addresses in @-form and qualified
them if they were unqualified on mail that originated locally.  If I may
assume that nobody is out to break it by putting at-signs into comments,
would this do?

 :0h
 * ^To:(.+$)To:
 tooz=| formail -zx To:
 :0E # not important to strip leading whitespace
 * ^To:\/.+
 { tooz = $MATCH }

 :0
 * 9876543210^0 ! ^To:
 * 1^1 tooz ?? [^       ]@([^   ]\.)+[^         ]
 * $ -1^1 tooz ?? $pattern
 action