Thanks for the reply @driz.
Without using SWAG, if I use the internal IP for the mail server it’s converted to the external address. With SWAG I get the certificate error.
Port 80 is open for MIAB (presumably to meet RFC specs) but redirects immediately to 443.
Thunderbird only uses IMAP & SMTP. Webmail is obviously on HTTPS.
MIAB is a complete mail server with webmail & DNS server and uses letsencrypt for the certificates, which is why I’m only looking for SWAG as a reverse proxy, and so that I can have a couple of docker containers internet facing. I can still get emails when I try redirecting through SWAG but I can’t get to the admin interface or anything but the main domain DOT com or box DOT domain DOT com, which obviously defeats the purpose.