Talk to your VPN provider whether they started blocking POP/IMAP, and SMTP protocols. Webmail is HTTPS, and has got nothing to do with it.

See https://nordvpn.com/features/split-tunneling/

Spoke with Nord, they were unable to give me guidance other than to try all the things I already had. They are not blocking POP/IMAP, and SMTP protocols. Also tried split tunneling and that didn't work either.

Spoke with Nord, they were unable to give me guidance other than to try all the things I already had. They are not blocking POP/IMAP, and SMTP protocols. Also tried split tunneling and that didn't work either.

Have you considered that Google may be refusing you? You have a VPN that is reporting you to be where i the world? And you have a phone without Nord that reports to to be somewhere very different to where NORD is dropping you off. What this is not is a Thunderbird issue. What this is, is a VPN issue.

It is quite simple, if it works without NORD, then the issue is not Thunderbird. It might be your antivirus, it might be your backup solution. But the only common factor you have identified is your VPN.

As I mentined in my original message, I have no problem logging directly into my Google accounts to get my emails with the VPN on or off. The VPN server is in NY, although I have also used double VPN to do the same without problem.

As I mentined in my original message, I have no problem logging directly into my Google accounts to get my emails with the VPN on or off. The VPN server is in NY, although I have also used double VPN to do the same without problem.

As you mentioned you have no trouble logging into a google web page and they choose to allow you to do so. You appear to be confusing your ability to access a google web page with email. They are not the same thing at all. That web page as much to do with email as the postal service has to do with Fedex. Both get things from one place to another but because one works it demonstrates nothing about the other.

In this case you have identified using the VPN causes you to loose the ability to send mail using mail protocols. You have also advised it doers not affect your ability to use a google web page which in the background accesses your mail stored on a Google server.

GMail has some very involved anti spam, anti phishing, and anti hacking protocols they use to protect your account. Some they publish, some they don't. Most of the emphasis is on automated connections as those are what hackers will be using, not some screen scraping software and a browser. So mail protocols using mail ports are heavily "protected" just as Microsoft does not allow the use of their Alias feature to those using SMTP, Gmail has it's own issues that have to be jumped over, avoided or simply lived with. Your issue is not as far as I can see anything to do with Thunderbird and you have not really suggested that it is.

Everything implies the issue is with the VPN as it's absence removed the issue, but if you want to investigate the process to determine the why. We could start with the error messages Thunderbird is offering you. What are they?

Then you might want to log the actual sending process (SMTP logging) so you can post the actual discussion between your system and the google system. https://wiki.mozilla.org/MailNews:Logging#Thunderbird_Logging_Types

So please log the process and post your log. No not redact random bits of the log file. It already redacts usernames and passwords and everything else is relevant. As you have a VPN, I will guess you also have a third party antivirus. Make sure it is NOT functioning when you undertake at least one sending test, as it may in fact be the underlying cause of your error. Their routine substitution of encryption certificates can be most problematic.

I do notice that the Google developer documentation says that gmail SMTP supports STARTLS although Thunderbird's default configuration only uses TLS.

So you might want to try SSL/TLS on port 587 or STARTTLS on port 465

I say try because that same developer documentation says IMAP and POP support SSL and Google deprecated that years ago in favour of TLS. (2015 vintage I think) So I have some qualms about the currency of their documentation. But perhaps using STARTTLS will force early encryption and may change something in how Google perceives the VPN.

BTW: Thunderbird uses SSL/TLS in dialogs, but you have to change default in hidden preferences to enable SSL at all. Any reference to SSL/TLS is actually TLS only, unless you have deliberately reduced your security by enabling SSL, and there are still mail servers exclusively using SSL only all these years after it was broken in the wild and folk who use them so the option to run using SSL is still needed by some folk.