What I did was set up a special user to receive the bounced mail, then run
a command every day that looks for bounces due to unknown user errors.
It's not perfect, but it has worked ok for me for a list of about 600.
Here's the cron entries:
00 23 * * * cat /dev/null > /var/spool/mail/trap
00 08 * * * grep 550 /var/spool/mail/trap > ~/dead.users
It keeps my mail spool small. The error number for unknown user is 550, so
you get a file with one line per bad message.
Bryan Blackwell firstname.lastname@example.org
'61 Corvair Lakewood, '65 Corvair Corsa, '66 E-type, '69 Road Runner,
'76 Ford F-250, '88 Acura Integra, '89 Camaro RS, '99 Neon R/T
"Why do something if you're not going to obsess about it?"