With virtualmin and postfix when using multiple domains, the imap etc for mail clients doesn’t work properly because postfix doesn’t support sni… It only works for the domain for which SSL is copied over to the postifx/devcot. A

To fix create a letsencrypt ssl cert for all domains or better just for mail.domain1.com and mail.domain2.com and so on and then use it for postfix/devcot.