How to enable Incoming mail to postfix on port 587?

[David Fletcher]

Hello,

I've been having troubles with the WiFi dropping out on my virgin
superhub 2 for a while so phoned them to complain about it. They've
sent out a hub 4 and a new TV box.

Today I'm aiming to get the hub 4 configured like the old superhub 2
before they send a technician here tomorrow. I've got it set to my
network address and DHCP range then started trying to get the port
forwards set up.

I've been using port 25 incoming to my postfix server for years and
it's always worked fine but the damned hub 4 puts up a message in red
saying that port 25 and a bunch of others are "reserved". What???

So far I've tried enabling port 587 and disabling port 25 on the
superhub 2 and making the adjustments I've found by Google searching
the problem to the master.cf file:-
changing smtp towards the top to 587
enabling the submission line
enabling the smtps line

So far cannot get it to work. Obviously I don't want to plug in the
damned hub 2 until I have confidence that email will be working when I
do!

Any further suggestions please? I'm running Ubuntu Server 20.04.3 LTS

Thanks everyone

Dave