KevinCCucumber
I told you already that nginx is only for http/https traffic (port 80 and 443).
STARTTLS is the switching of an unencrypted TCP connection to encrypted, and is used on ports 25, 143 and 110.
You seem not to understand how a mail server works, mail servers are one of the most complicated services to run and operate. mailcow might make the installation easy, but you need to understand how everything works together with the protocols on network level.
