Wireguard VPN - Mozilla Thunderbird times out

Connect AR300M16 Wireguard client to my home network AR750S Wireguard server. When I try to send emails, Mozilla Thunderbird times-out. I have no idea why this should happen.
Changed VPN to OpenVPN and all works perfectly, however I want to run my VPN over Wireguard.
Can anyone help me try to resolve this please.

Could you please ssh to the router, and execute this command? Please show me the result.