Basically your VPN is making a hash of network communications. probably in conjunction with an anti virus/ Internet security suite of software. Nord have spent very significant amounts of money on advertising campaigns that make some very questionable claims. Other that allowing the bypassing of geo blocking for US TV shows a vpn offers precious little to the average user. Why are you using a VPN?

So what mail provider are you using? I know of none that do not have some sort of limit, be it numbers per hour or day or bandwidth.

Are you even connecting to your server. Many anti virus product insert themselves into the sequence as a proxy. So Thunderbird thinks it is talking to say Google.com, but it is in fact talking to some rubbish software on your computer that is pretending to be Google so it can hack your secure communications and "scan" them. Made all the more ridiculous by the fact that if the product can't find malware on your machine it is not going to find it in your outgoing mail.

So you are using a very old version of Windows that will go out of support on the 14th of January 2020, so this is probably the last version of Thunderbird that will run on it.

You are using a VPN that has known issues with email. I keep posting this link https://support.nordvpn.com/Connectivity/1047408792/Why-can-t-I-send-e-mail-messages.htm

Thanks Matt. I do have Nord. And Win7 Ultimate. I also have Avast and Superantispyware. I use Frontier as my ISPm their outgoing server is brighthbouse. I had installed Ubuntu 18.04 but for some reason in going from version 16 to 18 they dropped "HDMI" out of the sound settings so I don't get any sound on my TV that I use for the monitor. The reason I started using Nord was because it sounded like a good idea. Maybe I should drop it. I wasn't really having any problems before that. I just thought I'd add a little more security. Maybe you're right. It used to be that if Thunderbird ever gave me that error, it just didn't send the email. I've had Avast and SAS for years and never had a problem before this. And when I first started Nord it seemed to be fine. Then I started getting these email errors.

Let me be very clear. Security is a compromise. Always. A compromise between usability and safety.

Over the years there have been a lot of folks that have slid that little slider up in their firewall and completely stopped everything going in and out because they thought it would be more secure.

I suggest you actually look to what it is that you are trying to achieve from your security and tailor it to that, rather than just paying for more because it will probably be better.

What is the benefit to you of a VPN? do you get a real benefit from brighthouse mail server being told you are connecting from say an ATT server? What about hiding your location from facebook? Most people gain nothing with hiding their IP address. Certainly they can be used to geo locate you, but most of the web site we access these days already know where we live (Facebook, Twitter, Google, Office 365) Even this site knows your email address and timezone, so telling it using a VPN that you are from Russia is not going to improve your personal security.

What VPN's are really good for is when you use a computer on the other side of the world to securely tunnel into your office network and use it like you are there. That is the true strength of the VPN. A connection to a secure network from the public internet that can not be snooped on.

What is the benefit to you of each and every function in your copy of Avast? Will your security be compromised in reality say if you turned off all email scanning? I would say no, Avast would probably tell you otherwise and we would both be right.

If mail scanning does no harm then it can be argued that it is another layer of security and therefore must be better and is a benefit to you. If it slows the process by 30%, which is common, or causes mail to just not go out, or to appear to be send when it is not then regardless of the supposed security benefits of having another layer, the practical benefit of being able to use your software makes any supposed security improvement null and void. (Anti virus product do not offer any more security by scanning email. All the threats that they can detect they should detect by other means if you have no email scanner at all. Hence Microsoft's decision to not have one in Defender.

One of the things you must also consider is that anti virus/ Internet security suite are updated daily, that can also include program updates, so a time will come when you will get an update that will mess things up. It is a simple matter of probability. So never discount security software when troubleshooting. It's reputation is really only as good as it's last update. Just as you are questioning Thunderbird. Everything is suspect.

Frontier has become troubling itself over recent years with vague email issues that come and go. They appear to have had some weird issues in the last 12 months, but I have not really seen them mentioned in this forum for some time.

In a completely off topic response. Your HDMI issues are probably solved by this article. https://itsfoss.com/how-to-fix-no-sound-through-hdmi-in-external-monitor-in-ubuntu/