Users typically don't get manually served off; dumb mailers do.  Users
who send transactions more than 10 lines long to LISTSERV rather than
the intended LIST get themselves defensively served off by LISTSERV.
 
Then they send it to the list and you get the message that the list
received mail from someone served off.  So you tried to serve them
back on and *possibly* forgot that the left hand side of the address
is case-sensitive or something obscure like that and LISTSERV told you
they weren't served off.  There's also the chance that the user noted
his entry didn't go, and was smart enough to get himself served back on.
 
If your served off as an automatic defense mechanism, *ANYONE* (except
you) can serve you back on.  If your local Listmeister serves someone
(usually a runaway automaton) off then I believe they stay off until
a local Listmeister serves them back on.    -Kary.