At present I get a “Thunderbird failed to find the settings for your email account” message.
I am presently attempting to work out whether the problem lies with:
1. Thunderbird
2. Incorrect router port-forwarding/filtering
3. A mis-configuration in my NethServer setup (hence question above).
I do remember being prompted to enter a non-standard port at some point in the setup - I now can’t remember where this applies though.
Plus one for @robb setup that works pretty fine also for Outlook 2013 (I hate autosetup of 2016 and 2019).
It’s mandatory to look for the correct hostname which receives the let’sencrypt certs: if certs are released for myserver.mydomain.tld the client should look for and correctly resolve, lan or wan, myserver.mydomain.tld
Many thanks @robb and @pike. I got it working in the end by giving the internal network IP as the hostname instead of mydomainname, and by allowing port 25 forwarding to the server in the internet facing router (still a little unsure about doing that).
@LeeT If you add the mailserver name to your internal DNS you don’t need to use the internal IP address. In fact, it would be better since you can also use the name when outside your network.
Actually it is quite easy: external: rely on other DNS services, ineternal: you are in control. Don’t forget to use your internal DNS server as resolver if you use it.