@TomMu 

Been many years since I set up and used a mail server. Some things come to mind.

reserve the MAC address of the PC in the router and restart the router.

Do you need to open and assign some ports? Port forwarding?

The firewall may be causing issues.

I seem to recall something with static address vs dynamic address could cause an issue. 

  @ArcherC8 

Thanks for for help.

I will try reserving the MAC address. Hopefully, this will allow the email server to be seen by the router.

I have a number of entries in port forwarding. All point to the email server's IP.

I use a static address for the email server.

The Archer 8 has the same firewall settings as the Archer AX80.

The email server works with my Archer 8 with no problem. Plus, I did not have a server problem when I changed routers to the Archer 8. The Archer AX80 is a different story.

  @TomMu ArcherC8. Thanks for your suggestions and help. It turned out that the Ethernet port on the router that I was using to connect the email server was bad. I switched to another port and it works fine.