Probably using some form of VALIDATE=.  BUT BEFORE YOU DO THAT,
please consider that this solution might be worse that the
disease i.e., forged signoffs are a rarity (so far) and probably
should not result in altering ALL subscribers' interactions to
signoff or other listserv subscription alterations.
 
What might appear at first blush to be a spoof, might in fact be
the attempt by a network or system administrator to adjust an
address for "local" reasons.
 
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