What is the best reverse proxy for Docker containers?

IMAP/SMTP works, depending on the server. You can't route to IMAPS/SMTPS, you need to do regular TCP. So either you manage IMAPS/SMTPS certificates on your mail server or you route to a non-TLS port.

Not only http/https, traefik supports tcp/udp as well, see https://doc.traefik.io/traefik/routing/routers/#configuring-tcp-routers.