- Plesk Onyx
- Valid SSL certificate is issued for example.com domain and installed in Domains > example.com > Hosting Settings > Security > Certificate, for example issued by Let's Encrypt.
- Another certificate is set for securing mail in Tools & Settings > SSL/TLS Certificates > Certificate for securing mail, for example, Plesk default self-signed certificate.
- Configuring a mail client, the domain name is set as Incoming/Outgoing mail server.
- The mail client is pulling the self-signed Plesk certificate instead of the Let's Encrypt certificate which is currently enabled for the subscription.
Such behavior is expected. There are two different certificates: a certificate for a website to use secure connection via HTTPS and a certificate to secure connections to mail server.
The connection is being established to the mail server and the certificate configured for securing mail in Plesk is used for the connection.
Change the certificate for securing email server with the one which issued for the server's hostname hostname.com using the instructions from the article.