Anyone who finds that Thunderbird stops sending / receiving emails when using Split Tunnel, but that it works OK with Secure VPN off, you should try this:-
If you can't delete Thunderbird.exe from Split Tunnel:- turn off Secure VPN. Locate the Thunderbird.exe file in Program Files / Mozilla Thunderbird. Temporarily rename Thunderbird.exe as, for example, Thunderbird1.exe. Add Thunderbird1.exe to Split Tunnel. Start Secure VPN. You should then find that Thunderbird1.exe works OK & sends / receives emails. Turn off Split Tunnel. You should then find it possible to delete Thunderbird.exe from Split Tunnel. In Program Files revert name to Thunderbird.exe. Add Thunderbird.exe in Split Tunnel. Start Secure VPN. Start Thunderbird. It should now work OK.