mailing lists

VERP is something djb's qmail does.  It's not probable that spammers
are going to run qmail.  Could happen but doesn't seem predestined.

Using a seperate envelope for every recipient is something that both
VERP (qmail) _and_ some common spamware both do.  But for different
reasons.  The point of VERP is to collect bounces and remove failed
addressed.  Spamware doesn't care about removing addresses, or for
that matter, failed addresses in any form.

Therefore looking for VERP's tracks as a way to deal with spam is a
form of "digging in the wrong place" (quoting Raiders of the Lost Ark.)