MX record error - mail server does not work

This is about the mail-server part of the community-server install. Install works fine, actually great, can access the portal but I can never get the mailserver to work. Opened and forwarded all ports etc. If it purely relies on port 25 then please remember that many ISP block that port by default - not a good idea to use it.
Any ideas, suggestions would be welcome (other than asking my ISP to open port 25).
After troubleshooting and a dozen (min) install attempts, I believe the problem is with docker but I cannot say for certain.

Would like to mention that I went through changes e.g. firewall etc. However, SPF ok, TXT record is ok but the MX record is not and I cannot send notifications which means I cannot invite users.

Port 25 must be open for the mail server. List of required ports:

Please make a screenshot of the error message you see and specify the version of Community Server you are using.

As for notifications, you can go to SMTP settings and reset to default so the notifications will be sent.