Found solution in another thread ... will copy-n-paste.
BOTTOM LINE: I changed server from ...
In a nutshell, I think even though portions of the auto configuration (some server settings and smtp) where acquired, the imap server address was actually being resolved with IPv6 somehow prioritized over IPv4(?). Upon modifying the /etc/default/grub file is described in this discussion: https://askubuntu.com/questions/1046057/disabling-ipv6-in-ubuntu-18-04, I was able to block IPv6 and its name resoluntion. Once I did this, updated grub and rebooted, ifconfig no longer displayed a dynamically assigned IPv6 address. I then opened Thunderbired, modified the IMAP account settings, entering Comcast's published imap.comcast.net server address and guess what? It worked.
In summary... If you're using IPv6 and trying to connect to imap.comcast.net and experiencing connection problems, you can either disable IPv6 use "imap.comcast.net" or change the server address to " imap.ge-xfinity.com". Not certain, but Thunderbird appears to beprioritizing IPv6 name resolution is prioritized over IPv4. In addition, Comcast doesn't appear to be aware of this and hasn't published the imap.ge-xfinity.com server address. Something has changed there at comcast and not every network admin/engineer may be aware; because the IPv4 & IPv6 host names and addresses are not synched to the host(s). It took an Ubuntu bug and the orginal post in this thread for me to derive this. ugh!
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re: Thunderbird email client
Now 03Dec2018 ... imap.comcast.net ... not responding for about a week.
Hours and HOURS on phone and chat with Comcast (Xfinity side and Comcast Business side).
Essentially no help ... no info.
Comcast personnel cannot tell me if they are having problems with their own Comcast IMAP server.
They keep telling me to contact the "Thunderbird Company" in order to correctly configure my Thunderbird client (running on Linux) ... which tells me they are pretty much clueless.
I keep asking them WHY I can "ping" Gmail IMAP server, Yahoo IMAP server, etc. ... but not the Comcast IMAP server.
I am told that the Comcast IMAP server "may now be a ping-less" ... i.e., maybe ICMP was set to not respond to a "ping".
I just checked comcast email via browser ... see a msg with subject "Test Email from Comcast Team" ... and it says ...
Good news! This message confirms that you can now successfully send and receive email via Comcast mail servers. "
Only problem is that my Thunderbird email client STILL DOES NOT WORK! Even if I follow instructions from their own website --> https://www.xfinity.com/support/articles/email-client-programs-with-xfinity-email
And ... I have not reconfigured my Thunderbird email client in year(s) ... so why did it suddenly start failing one week ago?!?
I am pretty fed-up with the level of Comcast tech support. :-(
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