Well here is my story.
Been using Thunderbird Email for about 3 years now. Upgraded to 68.1.1 but had issues with lower versions. This affects email only. All other Internet activities worked, browsers, download and streaming.
About July/August time frame I got this intermittent error: The server to imap.comcast.net has been reset.
Sometimes it would work for weeks, days and hours. Sometimes it would not. Just clicking Get Messages failed.
As this time I fell back to MS Outlook 2007 and it failed also!
I found a workaround by doing one of the following, and then TB and Outlook would work, but eventually fail again.
1. Switch to Airplane mode and back.
2. Switch to the 2.4G band.
3. Switch to 5G band and back.
4. Sometimes restart the modem.
TB would work days and then fail. I wait and it would work again. Comcast tried to say it was a TB issue but since Outlook failed the same way... can't be that. When email was working TB and Outlook worked.
Comcast came out and replaced the cable from the box to the house. It was defective but after this TB worked for about 2 days and then back to the error again.
Searching the Internet I found information on IPv6 and email issues. Folks said the disable IPv6 in their NIC cards and all worked.
Well, luckily IPv6 can be turned off in Thunderbird! So I turned off IPv6 and now Thunderbird works fine!
I can toggle the IPv6 off and on and duplicate the issue!
So, if you are having email problems turn off IPv6 in Thunderbird so it is IPv4 only.
Options/Advanced/General/Config Editor Except the risk.
Just enter IPv6 in the search bar.
Toggle the setting to read: TRUE
network.dns.disableIPv6 modified boolean true
Now your Thunderbird should work.
BTW... Outlook still will fail, but TB works! This is an intermittent problem with Comcast servers but can affect others.
You may have to turn off IPv6 on your router or NIC card in your computer especially if you use Outlook. This may be intermittent as Comcast appears to have implemented IPv6 incorrectly on some email servers. All depends on where you are and which email servers you get.