The Question is:
My question regards SMTP on UCX (TCPIP) v5.
Ideally I would like to listen on port 28 and send on port 25, but an
acceptable solution would be to listen/send on 28.
There does not seem to be a simple way to change from using the default port
25. I would prefer not to have to reconfigure the SMTP service from scratch.
The Answer is :
You could fairly easily change the port number for SMTP in the service
database, or add a second service listening on a different port number.
That, however, would only affect incoming mail.
For outgoing SMTP mail, the remote port number of 25 is hard-coded, as
it is a fixed architectural constant for all SMTP implementations. The
local port number used for outgoing connections is not hard-coded, but
will vary, with a new number generated for each new connection.
When you think about it, you really wouldn't want the system to use
port 25 (or 28, for that matter) at both the local and the remote end.
If it did, you could never have more than one SMTP session at a time
between a particular pair of hosts, and you'd have to wait 30 seconds
after closing that connection before you could open a new one.