I updated recently to a new version and discovered the SMTP setup was using self-signed certificate (I was using Lets Encrypt). However, everything else was using the proper certificate. Even after reissuance, it was still not fixed. Turns out the real certificates that are being used by qpsmtpd are located in /opt/qpsmtpd/ssl, not in /etc/qpsmtpd/ssl. Maybe this should be fixed in the next versions?
Issue #144 closed