as an fyi - I'm going to add on to what I started and extend by php program that parses the /var/log/maillog file to do it daily, keep track of the number of emails sent to each member and the number of bounces sent to each member.
If a forum member has a bounce on 5 different days then I'll set their email address to blank. This way I can avoid setting their email to blank if they get just one bounce. This is beause there are often legitimate bounces such as soft bounces because their inbox is full, their service provoider's email server may be offline etc. Therefore if we cannot contact them on 5 different days (over any span of time) then we'll clear their email address.
HOWEVER - if we do deliver an email successfully, then thy bounce-count will get reset to zero and the bounce count starts all over again