It is quite common for mail servers (especially those offered by ISP's) to not function when a VPN is being used to mask the service and IP used and set by the ISP.

Does you mail provider accept outgoing mail if you are not connected to the internet using their service? Many do not and a VPN makes it look like that.

Does your VPN provider even support email? Again not all do, and some require all sorts of special settings to make mail function on their VPN.

Finally anything web browser is not mail. It is a web site showing you your mail and sending it on your behalf. It is not a mail client, despite the similarities. As your VPN will be set up to work with web sites then web mail works.

Do you even have a VPN on your android devices you are comparing with?

Thanks for your answer Matt.

I don't quite understand what my ISP has to do with this issue. I've been using a VPN for years now, the only new element in my setup is me switching this week from NordVPN to Surfshark, so the problem I describe, I dare say, is not linked to anything else. It's only an issue on my computer using the Thunderbird app when I switched VPN.

And yes, I'm using Surfshark on my tablet and phone to compare : I can send mail fine since I'm not using TB.

The error message says it all. The Server is refusing the connection and this is likely to be because it thinks you are not you. It thinks someone is trying to abuse your email address and send through your account. It's possible the server has detected several different IP addresses trying to access the server.

ISP prefers you to have an IP address they can keep track of, so if you had NordVPN set up to use a dedicated / static IP then ISP server would see this as ok because it's not being shared with a load of other people.

Is SurfShark set up to use Static Server so you get same IP each time ? Useful link: https://support.surfshark.com/hc/en-us/articles/360012451580-Surfshark-Features-and-Settings