As an owner of SAS-L (along with upteen other lists), if several
rejections from a user come into my box over 2 days, I delete the
person.  It may not be fool proof, but I don't have the time to spend
tracking down each postmaster, and asking if the account is in fact
defunct.  If your system is sending back messages saying unable to
deliver for one of ten reasons, one of which might be box full
or another is account deleted, I will delete that person.
I'm using the LISTDEL XEDIT that I offered on this list a while
ago, and several others are also using.
 
Harry