I know .... again this problem is posted. As this is my first post here, may I say so far I'm very impressed with the 3CX offering, especially at the 'free' level! I installed the latest (15.5) Debian ISO version and the setup went well until the outbound email setup. If someone could give me a couple clues maybe I can fix it myself. I think I need to change something in the 3CX Debian config. Question - what system is used to send email? I don't see Postfix installed. I don't see PHP installed so I'm not sure how to diagnose the issue. Where are most detailed logs kept? Specifically, I would like to use my own email server which currently works great with Mac, Windows, Linux, and iOS email clients. But not with 3CX email client trying to send via secure SMTP. In the 3CX setup I have my server specified and submission runs on 587 as expected. When I try the test email button 3CX says "The remote certificate is invalid according to the validation procedure." And my email server says: Aug 27 18:52:14 myserver postfix/smtpd: connect from <my3cxbox> Aug 27 18:52:15 myserver postfix/smtpd: Anonymous TLS connection established from my3csbox.: TLSv1 with cipher ECDHE-RSA-AES256-SHA (256/256 bits) Aug 27 18:52:15 myserver postfix/smtpd: lost connection after STARTTLS from <my3cxbox> (some details redacted there) My server uses letsencrypt 4096 bit key, so maybe that is not compatible. I tried lowering my minimum cypher security level from high to medium but that didn't help either. Postfix on my email server is serving the letsencrypt fullchain.pem cert. 3cx seems to be aware of letsencrypt since it uses their certs itself. Thanks for any help.