What's the security/AV program, and it's email and SSL-connection scanning settings?

I'm using ESET. There are a ton of setting. Care to narrow the list?

Also, I've tried disabling the whole thing and no luck.

And, lastly, the error window is via Thunderbird, not ESET.

The problem might not be caused by ESET, but a simple test is to run Windows in safe mode, bypassing startup apps like ESET, then run TB.

What are your smtp outgoing server settings?

The SMTP settings are as directed by my host. Port 587. STARTTLS. Normal Password. Etc. Other email accounts on my mail server NOT using thunderbird are not having the issue.

They seem to think it's a Thunderbird bug. I tend to agree.

When I originally updated Thunderbird - ALL email stopped. It wasn't until I changed the min.security.tls value was I able to download them.

It would be helpful to understand just what the heck the error means. Inadequate?

I'll attempt the safe mode later.

OK. Apologies for the delay - work gets in the way.

Upgraded to v78.5.0 in the hopes this was fixed. It was not.

One point of note before I go further: I have 3 accounts in my local Thunderbird: Company A, Company B, and my wife's Optimum Account. I have SMTP servers set for each one - and have been using her Optimum to send. I can receive emails fine on all 3 accounts. For the Companies' servers, each has it's own server - mail.<company>.com - and each POP/incoming/receiving is Port 110; SMTP/outgoing/sending is Port 587.

I noticed there is certificates for each server at Port 110. Only Company B, however, has one for Port 587.

So, I tried Safe Mode with Networking.

When I tried to send from my Company A account to my Company B account, the send window just got stuck on "connecting to mail.<company A>.com server." Forever. 5+ minutes. When I hit cancel, it said the connection was terminated in progress.

When I tried to send from Company B to Company A, the original "inadequate" message returned.

Arguably, this means it is not my AV?

So, what now?

OK. Updated to ThunderBird v 78.5.0 in the hopes the bug was fixed. IT wasn't.

I then looked into Thunderbird and my certificates, and noticed certs for servers mail.fenrir.com:110 and mail.echelon-us.com:110 under a heading echelon.nerivon.cloud.com (the certificate names was also that echelon.nerivon.cloud.com.) There was another cert for server mail.echelon-us.com:587 under a Echelon Materials LLC heading, also named echelon.nerivon.cloud.com.

There was one last for a server mail.fenrir.com:587 under a webair.com name and server. I deleted that one.

I have not been able to add a mail.fenrir.com:587 cert at all.

I then restarted into Windows Safe Mode with networking support, launched Thunderbird, and tried to send an email from my RLM@fenrir.com address to my bob_muller@echelon-us.com address thru the mail.fenrir.com sever and port 587. The "connecting with mail server window popped up and stayed there. 5+ minutes. When I cancelled it, it said "server terminated communications before completion."

I then tried to send an email from my bob_muller@echelon-us.com address to my RLM@fenrir.com address thru the mail.echelon-us.com sever and port 587. That returned the same "certificate key usage inadequate" error.

However, is it safe to conclude that it is not my ESET AV getting in the way as in safe mode it was not active?

What next?

UPDATE: Finally got hosting service to re-install / re-issue the certification on the serrver and, blammo, all works.

Problem solved