[tardisx]

You can just copy the caddy-generated certificates to the place the mail server expects them, if you're referring to the TLS side? This is what I do.

[elitepleb]

and https://maddy.email/ is built with a similar spirit

[Avamander]

Stalwart is probably more feature-complete than Maddy by now.

[cuu508]

Perhaps I missed it but doesn't look like it supports LetsEncrypt (maddy does!).