tried disabling your anti virus from the scanning mix. they are notorious for messing up mail connections, dropping them, requiring special certificates and generally being of poor quality.

When troubleshooting one should always disable virus/malware software along with any firewall software/hardware first. Also disable any browser extensions or add-on's. Windows defender has always played well with email, at least for me. I also use Trend-Mico with no mail issues. Generally speaking, I never advise my customers to pay for virus/malware as there are free products that work quite well for the average consumer. I simply take extra precautions because of my business environment. Thank you so much for your response and concern for my issue.

What is most interesting, was after I formatted my drive back to the previous weeks image where everything was working, yet the problem persists. That leads me to believe something has changed within the Thunderbird email client itself or Comcast/Xfinity has changed something.

By no stretch am I a fan of Comcast, they are unfortunately my only alternative for internet access. It is a love/hate standoff. I never get the up/down throughput I'm paying for but it serves its purpose. That is a topic unto itself for discussion later.

I have managed to get two gmail accounts back, my windows live account and my domain account which has two mails, all are up and running. I had to change a couple ports along with authentication and security settings. Its as if Comcast has rolled over and died. No big loss really as this has been more of a nuisance than anything. Life goes on...

I just read that you use a VPN, I missed that in my first trip through.

That is a frequent issue with email as some of the less helpful ISPs require a connection through their network to send mail. The end point IP of the VPN is rarely in their network. But Spectrum is the 100 pound gorrilla in that room.

If memory servers comcast use different servers for mobile/cell devices and you have to use those also with a VPN. but it is just a niggle in the back of my mind.

Matt, you are quite correct in regards to using VPN, dealing with the isp their servers, mail and the cell network. I did test send/receive mail via my cell phone all worked with no issues.

Ah Spectrum & Charter, I find no good in either! I have been "dealing" with Comcast for 10 years since moving from Atlanta to south coastal Georgia, 
I know what to expect or what not to. I use their internet for the simple fact, its better than at&t which is my retirement employer. I do not use or trust Comcast hardware, their router/gateway has a built in wifi hot spot feature. I'm not paying a monthly fee just so they can provide a hot-spot on my dime. CC alleges it can be disabled.    

My condo environment influenced me to use a VPN. the cost was minimal. My throughput suffers slightly but so long as the wife is happy with her devices, life is good! I average 70mbps down and 20 up, depending the weather.

My problem is "halfway" fixed, so to speak. I can now receive Comcast mail on Thunderbird, cannot send. I setup Comcast mail on W10 mail client, simply to see if it worked, it did. Its easier to use my cell .

I have tried every setting, the old saying, six ways from Sunday, to no avail...There is a fix...it's in front of me, just cant see it....yet...I will sleep on it.

TomD said

I setup Comcast mail on W10 mail client, simply to see if it worked, it did. Its easier to use my cell .

That to me indicates one of two possibilities. Anti virus springs up because they only appear to test on Microsoft products. Thunderbird connectivity is frequently held captive to the vagarities of the ubiquitous smelly piles. Particularly their firewalls. But we do recommend turning off email scanning in anti virus products if you are having difficulties or experiencing slowness.

There is also the worrying trend of anti virus companies "fiddling" in the certificate store and making themselves a certifying authority. They don't go through the audit, just rely on user trust and ignorance to allow the changes. Because they can "just" update the windows store, Microsoft products work fine with their hack. Mozilla products do not as they use their own certificate store and manual intervention is required to get their certificates into that store. This usually manifests as a certificate error with an unknown issuer.

Then we have the final hurdle. The cipher suites and encryption protocols used. Microsoft products are usually months behind Mozilla and Google is updating their software to not use compromised protocols and ciphers. For instance Norton/Symantec made such a hash of things their SSL/TLS certificates are no longer supported in Mozilla or Google products. but Microsoft is dragging the chain. (less consumer support required if you don't fix those sort of things quickly. )

.

I've tried once more turning off TrendMicro & the Firewall, turned back all settings to default, same outcome, Comcast mail will not send. I also went through the list of "allow apps & features" through the firewall settings, public & private, mostly default settings. I can also ping all ports on my Cisco router.

I do not get into the certificate thing so much other than knowing how & why they do what they do and where to look when needed to make changes if a situation dictates.

For the time being I'm tired of fiddling with it, I've wasted to much time pursuing it. Thanks for your input, it has been most helpful!