- Plesk Onyx
Email client connecting to the mail server configured by Plesk display expired SSL certificate error
Configuring a mail client, the domains name is set as Incoming/Outgoing mail server.
Valid SSL certificate is issued for example.com domain and installed in Plesk > Domains > example.com > Hosting Settings > Security > Certificate
Another version of the certificate is set for securing mail in Plesk > Tools & Settings > SSL/TLS Certificates > Certificate for securing mail.
Outdated SSL certificate was used to secure email
Configure email server with an actual SSL certificate.
Refer the following KB article to install SSL certificate to mail server:
How to secure a mail server with a purchased SSL certificate in Plesk