From what I understood, SJU's name server occasionally goes berserk and
issues authoritative statements about domains it normally has no
authority to speak for, saying the host does not exist. This results in a
permanent "no such host" failure which causes SMTP to bounce perfectly
valid mail and report it as a permanent (550) error, which in turn causes
users to be deleted from lists. If the incorrect information comes from
SJU's name server, that's where the finger will point when people
complain about lost mail. If you can't fix the name server, the next best
thing is not to use it.
Eric