I've corresponded about this with Charles Orth ([log in to unmask]), who is
 involved in maintaining the mcimail gateway, and, yes, it is definitely
 mcimail's problem.  Associated with this is the fact that if one of your mci
 subscribers cancels his/her account, mail to all your mci subscribers will
 bounce until you delete the non-existent user.  I suggest writing to Charles
 if you want more definitive info.  He has been straightforward in
 acknowledging both the problem and his lack of resources to deal with it. --
 Phil Sollins