If you have a mailing list which is generating a bounce message, and the bouncing address is not obviously a string on your list, there are a few things which you can to to diagnose the problem.
The rblanche command in the consult locker will recursively list all members of a list, including members on sublists. If you use the "-c" flag to rblanche, it will also check for any chpobox'ed mail forwarding on the accounts (this slows it down, so the -c flag is not enabled by default).
This would indicate that the list "mylist" contains four users plus the list "myotherlist" and that the list "myotherlist" contains two users. Additionally, the user fred is forwarding his mail to email@example.com, and the user jill is forwarding her mail to firstname.lastname@example.org; the user peter has split his forwarding to go to both his athena account and to email@example.com.
Note that if you have an address @alum.mit.edu which is on your list (or someone is forwarding their mail to their alum account), there is no way for you to see where that mail is forwarding, because Alumni Network Services does not permit lookup on their mail hubs. However, if you believe that an address at alum is generating your bounce, you can send a list of the alum addresses on your list to firstname.lastname@example.org and ask if any of them are pointing to the address which is bouncing.