First, you aren't required to use Thunderbird. Second, the issue you're addressing is not caused by Thunderbird, but by your email host's server, which is seeing a conflict in the IP addresses in the email (which is caused by the VPN). Thunderbird is just reporting that error to you. That's the role of a good email client, serving as your agent. You might see if the VPN can be set to not interfere with email clients.

Firstly. I am well aware that there are a variety of email apps that I can use. Secondly I was quite happily using Thunderbird up until the most recent update when the issue first appeared. Thirdly there is a plethora of similar complaints regarding this particular issue (no longer able to send an email). Your answer does not address the issue that the latest Thunderbird update is responsible for this issue appearing. Re the VPN, Thunderbird has apparently happily managed the VPN issue in the past. I was able to pin down my problem but there are thousands who have no idea why Thunderbird suddenly stopped sending emails. In my case, all I received was a garbled BS message saying only that Thunderbird could not send the email. For someone not IT savvy, Thunderbird no longer works and they would not have a clue why or what to do next. Your supercilious reply would mean nothing to them. Try to be a little more understanding.