Unable to add mailbox to mail client: unable to verify the mail server identity

Symptoms

• Connecting a mailbox to a mail client results in the following warning:

  PLESK_ERROR: Unable to verify the mail server identity

• Default certificate is set in Plesk > Tools & Settings > SSL/TLS certificate > Certificate for securing mail

Cause

Default self-signed SSL/TLS certificate was set for securing a mail server.

Resolution

Change SSL/TLS certificate to either a free SSL/TLS certificate from Let’s Encrypt or an SSL/TLS certificate purchased from a certificate authority with reference to the article:

How to secure a Plesk mail server with an SSL certificate (Let’s Encrypt / other certificate authorities)

Additional Information

How to configure a secure connection when setting up an account in an email client

Securing Plesk and the Mail Server With SSL/TLS Certificates