resolv.conf problem

[Jim]

Today I noticed a problem when I use Thunderbird to check my email on my 
Kubuntu 18.04 box.  I got  the following error in Thunderbird: Failed to 
connect to imap.comcast.net  However it will connect to 
imap.comcast.net.  I also couldn't ping imap.comcast.net.

delboy at ladmo:~$ ping imap.comcast.net
ping: imap.comcast.net: Name or service not known
delboy at ladmo:~$

In Network manager I have  the following DNS servers in this order:  
192.168.1.4 192.168.1.1, 8.8.8.8 8.8.4.4.  I found this in /etc/resolv.conf

# Generated by NetworkManager
nameserver 127.0.0.53

If I change 127.0.0.53 to one of the DNS servers I have in Network 
Manager, Thunderbird works again, but I'm not able to ping 
imap.comcast.net. I get the same error I mentioned earlier.  I really 
don't care if I can ping it or not as long as Thunderbird can connect to 
it.   However when I reboot or restart Network Manager, resolv.conf goes 
back to 127.0.0.3.

Is there some bug with Network Manager or has comcast manged to screw up 
something?

thanks