Just to interject my $.02 worth, I do approximately the same thing.  I
watch for repeat occurances of a given failure, after several repeats I do
send test mail, and if that fails also, I remove the user.  This holds true
for BITNET sites, but I do things a bit differently for Internet sites.
Depending on the error that is returned, I ignore it, especially if it
pertains to any sort of mail/system failure.  If it comes out with an unknown
user error, I usually treat it the same way as the BITNET failure.
 
Rich