I own some 40 lists (only a few are very active at any time, luckily)
and here is some of my management tips (some of them are "ruthless" for public lists!!)
 
I put all error messages in a folder (Windows95), check them every few days, and whenever I see continued problem cases
(1) I change options of "disk quota exceeded" to "digest"
(2) delete  any  "unknown user" if the autolistserv service gets them first (I use QUIET DEL *  address)
(3) delete any problem cases even if I dont understand the message in the error messages (some of them are just a waste of the bandwidth).
 
 
rgds, jacky