Quoting "Stuart D. Gathman" <stuart(_at_)bmsi(_dot_)com>:
Why not? Have users set SMTP port to 80 (or 443). What is the big deal? Is
it harder to type than 25 (or 587)?
Users don't generally type a number there at all. That's the whole point: we
need USER-INVISIBLE solutions.
With sendmail, you can listen to as many ports as desired, on any specified
IPs or ANY using a DAEMON_OPTIONS config line for each port.
I presume that any decent MTA would have something similar. If your
web server is on the same machine as your SMTP server, have the SMTP
server listen to port 80 on a different IP address.
Don't you think that any environment that blocks ports is also likely to use a
web proxy and block any direct access to ports 80 and 443 other than to the
proxy?