probably the best way to handle the "travelling user" need is with
smtp-auth -- where the sending client authenticates.
look at/for the following lines in your sendmail.mc
dnl # The following causes sendmail to additionally listen to port
587 for
dnl # mail from MUAs that authenticate. Roaming users who can't reach
their
dnl # preferred sendmail daemon due to port 25 being blocked or
redirected find
dnl # this useful.
the concept of temporarily adding a random/dynamic ipnumber isn't
very good as it open a (admittedly probably small) window that might
be able to be used by a spammer type should they come upon it.
Thank you very much for the help! I uncomment the line
DAEMON_OPTIONS(`Port=submission, Name=MSA, M=Ea')dnl
but, I have a question: if outlook express does not use port 587, then what to
do? How to make sure that if outlook express finds port 25 blocked, it will
try port 587?