Well, if you can't deliver mail because their mailbox is full, then you should simply wait a few days before trying to deliver mail to that address. If it's because the address isn't there any more, you should delete it from the subscription lists. Otherwise, the instant people's mail quotas overflow (e.g., they're on AOL and they haven't been able to get through the clogged lines), they get bounced... or you're left with a lot of invalid addresses. The address causing the bounce is not necessarily determinable automatically from the bounce message; if it isn't, then you need to keep a copy around for a human to look at.
who should bear the burden of dealing with bounces? the list admin or the owner of the bouncing address? if you have the time and desire, you can accomodate people that bounce mail due to a temporarily full mailbox. if not, unsubscribe them after N bounces in a 24 hour period. it becomes their responsibility not to bounce more than N emails. when a list has 1000's of subscribers, i have not have been able to provide the type of personal service that the first option entails. i do make a digest version of every list available. if the digest bounces, the bouncer??? bouncee?? stays below the N bounces threshhold (for many values of N :). jmb