I tried this but it’s not working on my site. So basically I am using the default nginx and doin the ssl offloading with Traefik

You can simply use traefik as ssl offloading pointing to the nginx container in the lemmy stack. That’s what I am doing with docker swarm. I have not figured out how to get rid of that additional nginx

Running a mail server these days is not that difficult. While using pre-assembled stacks like mailcow only the DNS entries needs to be done. If you want to run it at home you should do some research on routing all the traffic through a wireguard tunnel to preserve a public IP other mail instances will accept