Today (Nov 29, 2018) I found that my two UM 18.04 machines would not connect to Comcast mail servers using Thunderbird. The messages made it look as if account settings had been changed, but I verified that they had not. After a long, involved process I have narrowed the problem to a network issue. I have found that if I use the UM “network settings” dialog to set up Cloudflare DNS (1.1.1.1), then Thunderbird connects to Comcast just fine. (My router was/is already set up to use 1.1.1.1 as my DNS rather than my ISP.) I can also use a VPN to get to the Comcast mail servers successfully. Unfortunately, either solution requires attention with every log-out or restart.
My default configuration with stock UM settings and local router set for a DNS of 1.1.1.1 or even the local ISP’s DNS fails. My router’s firmware as been static for a few weeks.
Another interesting piece of info is that another machine on the same local network has not experienced the problem. It is running stock Ubuntu 16.04.
I am wondering if anyone can offer some insight into this situation. My thanks.
One day later, I believe that I have found a solution thanks to the info at https://askubuntu.com/questions/1012641/dns-set-to-systemds-127-0-0-53-how-to-change-permanently . Briefly, I did the following to address the problem.
installed resolvconf (sudo apt install resolvconf)
restarted
edited the file named tail found in /etc/resolvconf/resolv.conf.d/ by adding entries like these for the Cloudflare DNS. . .
nameserver 1.1.1.1
nameserver 1.0.0.1
restarted again
The results is that the new DNS servers are made permanent and can be seen using the network app. Now I should learn why this worked!