If I recall, the RFC821 MAIL FROM:<> field userid portion has a
"fingerprint" so that if the target mail system "bounces" to that
address, LISTSERV(R) will then use the fingerprint to ID the bouncer.

I don't recall the format, but to give some concreteness I could imagine
a probe on my subscription would be something like (say on LSTOWN-L)

MAIL FROM:<[log in to unmask]>


At 20:57 10/7/98 -0500, Anne H Toal said:
|What does Listserv insert into a message to make it a "passive probe"
|(1.8d)?