Hmm, NOrd are spending a lot of money advertising and little money on actually telling folks that their service is not actually email friendly.

Have you asked nordvpn? https://support.nordvpn.com/Connectivity/1047408792/Why-can-t-I-send-e-mail-messages.htm

I have no idea who your host is, but if it is one of those offering exchange email then outlook will not be using IMAP or POP mail protocols or standard mail ports at all so the comparison is less useful that checking web mail.

I've tried Nord's basic suggestions (change port, disable Cybersec etc.) and they don't help. I have discovered that switching to another VPN server allows me to send, but I do need to use my static IP. I have sent a support ticket outlining the situation to Nord - we'll see what they say.

However, I'm sceptical that this is a Nord issue, since Outlook sends fine over the static IP. Thunderbird seems to be the factor here. Web mail sends fine. I really do not want to switch to Outlook!

Any ideas? Anything else I could try?

I am tired, very tired of trying to explain to people that offering anything about web mail not being affected is about as useful as me posting this link. https://en.wikipedia.org/wiki/Main_Page

Outlook will be connecting to the server using the proprietary Microsoft exchange protocols is there is an exchange server available. So using outlook as a possible working model is like web mail entirely useless information that most folks try and use and spend ages chasing their tail because the are unaware that their comparison is false.

Sure outlook gets mail, but that is where the similarity ends, unless outlook is configured to use the same ports, protocols and encryption Thunderbird is using. It doing something or not is entirely irrelevant.

Want to test your theory properly. get a mail client that like Thunderbird connects to the server using IMAP pop and SMTP. Then you will be comparing apples with apples. Something like https://www.ritlabs.com/en/products/thebat/

You still have not identified the server your trying to connect to. But it probably has some form of geo blocking on access, so your static IP is in the wrong geo location for your provider to accept it is you. Many US based ISP's block access to their mail servers unless you connect to the internet from their service (Something a VPN is supposed to hide) you can not even use free wifi hot spots, unless they are provided by your mail provider.

When I tested sending over VPN and static IP with Outlook, I configured the account to use identical SMTP settings to that in Thunderbird. Can I clarify (as I'm not a mail expert) that you suggest that this proves nothing?

I am trying to narrow down the issue by comparison. I guess I'll try some more dektop mail clients, configure SMTP exactly as Thunderbird, and see if that establishes the case that this is a Thunderbird-specific issue.

I am well aware that webmail is irrelevant to SMTP - I just wanted to make the point that I can be sure there isn't a fundamental problem with the account.

Hi Not sure if this will help you - but may be worth eliminating from your trouble-shooting. I do use VPN, and I am behind a static IP, also am connecting to an Exchange server. I find that in some circumstances if I am connected to one service on the server, and then I try connecting another (one on VPN and one through browser), then the server will block one connection. It's seeing the simultaneous connections as a security issue. To further complicate the question - if it is an Exchange server, depending on the settings the server allows, Thunderbird may need an intermediate add-on like Owl or Exquilla helping to connect using the proprietary Exchange settings. Exchange servers do not normally permit standard SMTP/IMAP/POP. (As Matt mentioned - Outlook does not need those extra add-ons as it is also MS product.) So find out if you are connecting to a MS Exchange server. If it is not Exchange, then you can ignore the second part of the above. Agnes

I installed Mailspring. When setting up my account, I was presented with a username/password authentication error for SMTP, until Idisabled my static IP. Then I could set up the account. Once the account was sent, I sent a message (OK), then enabled my static IP. On trying to send again, I received an authentication error.

So yeah, maybe this isn't a Thunderbird issue.