If an ISP will firewall port 25 due to spamming concerns, what is
stopping them from firewalling port 465 as well, besides lack of clue
and/or lack of actual use of the port?
25 is used for message relaying in the absence of any explicit agreement
between the parties. the ability to authenticate cannot be assumed.
therefore, if you block 25 on a certain path you need to have reasonable
assurance that you won't be blocking mail that you want to send or receive.
465 is intended for message submissions. because it is a new port,
it's reasonable to require authentication. and because authentication is
required it's far less likely to be used as a means of relaying mail from
unauthorized parties. also, having a separate port for message submission
provides an opportunity to provide additional processing that is appropriate at
message submission (like rewriting message headers to add missing domains,
dates, etc., and/or rejecting messages which aren't perfectly formatted) but
not appropriate for mail relays.
of course a network can block 465 if it wants to do so. but often it's useful
for a network to block outgoing traffic on port 25 but unnecessary for that
network to block traffic on port 465.
Keith Moore http://www.cs.utk.edu/~moore/
27 February 1933 11 September 2001