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