No idea, the limit is imposed by your provider, as you do not mention who that is I can not google to get the information for you.

My isp is TalkTalk.

Google search on talk talk indicates they will not advise what their limits are, it is for the customer to guess. Apparently by denying you certitude that can pretend they are denying the information to spammer. Pretty flawed logic as spammers just don't use mail provider SMTP servers and haven not done for decades. But it sounds good. I assume talktalk are still operating on last centuries standards.

There appears to be a general consensus the limit is somewhere around 30 per email.

Sorry but there is something not right. I sent the email via outlook to the same SMTP server and all was ok. This would tend to indicate the problem is within Thunderbird. Now, when I try to send any email I get another error message (screen shots included). When I enter the password it shows the msg it shows Enter New Password. When I do enter new password it loops back to error msg. The email is still in process as you can see from screen shot 4 and just sits there with the green bar moving; nothing happens.

I am having real problems now. I have uninstalled Thunderbird and re-installed it to see if that would resolve my 'too many recipients' problem, but it didn't. Now, I can't send an email with one recipient!!

Outlook uses the windows certificate store and Thunderbird it's own. What anti virus do you use?

I use Birdefender as my virus protection, but I figured out the problem. The problem occurs when I am running the bitdefender vpn. Stop the vpn and all is well.

So the problem is your mail provider is refusing to send mail as you are not connected to the internet using their service. Sounds like simple security really.

You must ask yourself what actual security the VPN is providing. My personal feeling is next to none. Most folks are connecting to google and social media that already has all their details of who they are where they live because we gave it to them. We also told them who our friends, family and colleagues are depending on the service. They all get telemetry back from all those little little social media "share" buttons that are on almost every web page and if you use google ad-sense the code in the page contains additional feedback to google them what pages we have visited when.

The short of it is a VPN is wonderful if you want to log into the office network from home.. an encrypted tunnel across the intent to another private network, very clever... a general VPM. Your communications are encrypted by the VPN until they leave the VPN, which they must at some point because you need to connect to general internet sites like gmail, facebook Twitter etc. Your connection speed is generally slowed as everything has to route to and from the VPN and then be encrypted for transmission though the VPN and then unencrypted. That is in addition to the encryption used in all HTTPS connections. The main benefit if VPN's is to disguise your location based on your IP address. Handy for Australians that want to watch geo-blocked content on US TV networks web sites. Nasty if your ISP uses your IP address to decide if you are you and allow or decline access based on that determination.