Ok. I have spent the last 3 days to get this to work so you best gimme some kudos Create new sites on ISPconfig 3.1 "perfect server" (in this example we call these new web sites ic4.eu, smtp.ic4.eu and imap.ic4.eu) and enable Let's Encrypt SSL on these sites. Use www. as alias on ic4.eu root (but remember to disable www. alias on any other web site that possibly will use the (ic4.eu) root domain address like smtp.ic4.eu or any other web site alias in the future.) Make sure your DNS settings have these new addresses. Define ISPconfig to use the new SSL certificate with symbolic links. (If you don't know how to use symbolic links this how-to is not for you) /usr/local/ispconfig/interface/ssl/ ispserver.crt -> /etc/letsencrypt/archive/ic4.eu/fullchain.pem ispserver.key -> /etc/letsencrypt/archive/ic4.eu/privkey.pem Define Postfix to use the new SSL certificate in /etc/postfix/main.cf. (If you don't know how to add these entries this how-to is not for you) smtpd_tls_cert_file = /etc/letsencrypt/archive/smtp.ic4.eu/cert1.pem smtpd_tls_key_file = /etc/letsencrypt/archive/smtp.ic4.eu/privkey1.pem smtpd_tls_CAfile = /etc/letsencrypt/archive/smtp.ic4.eu/fullchain1.pem Define Dovecot to use the new SSL certificate in /etc/dovecot/dovecot.conf. (If you don't know how to add these entries this how-to is not for you) ssl_cert = </etc/letsencrypt/archive/imap.ic4.eu/fullchain1.pem ssl_key = </etc/letsencrypt/archive/imap.ic4.eu/privkey1.pem Enjoy your working SSL certificates for SMTP(s) and IMAP(s). Who's the man?