The errors are probably from one of two sources:

1. Subscribers who have their email forwarded to another address 
which is bouncing.

2. Spammers trying to send to your list and getting rejected.

Add Probe(1) to your Auto-Delete setting for a day to flush out those 
of the first type. This forces a passive probe on all subscribers for 
the first message of the day, and will cause bounces at forwarded 
addresses to be counted against the actual subscribed address. (Take 
the Probe(1) out once that's been done, because probes require more 
resources). 

As to the second type, just ignore them. They aren't going to go away 
-- be thankful that they are getting rejected instead of being 
distributed on your list and that the only sign you see of them is on 
your daily error monitoring report. 

Francoise