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