This isn't a Thunderbird issue. Email hosts check ip addresses to prevent spam and VPNs interrupt that process.

I have this problem, it is a Thunderbird/NordVPN compatibility issue since the latest T'bird updates. I have had the NORDVPN team remotely log into my PC and they have stated this is not a Nord Issue it is a T'bird issue. David's comment about it being an email host IP check issue is not accurate because I can send emails to the same addresses using Outlook or if I log into accounts online.

Interestingly, if I change NORD to use a US based server, it works fine!!