Question
How to configure a secure connection when setting up a Plesk email account in an email client?
Answer
Note: To secure mail for a domain, refer to this KB article.
To set up an account with secure connection to a mail server in an email client, follow these steps:
Make sure a valid SSL certificate is assigned to the mail server in Plesk at Tools & Settings > SSL/TLS Certificates and domain's name in the certificate is the same as the name of Mail server.

Click on the certificate name to find out for which domain name it was issued.

When a valid SSL certificate is set up for a mail server and domain name is known, during configuration in a mail client, specify incoming and outgoing mail server name as domain name from step 2.
Incoming server
IMAP - use port 993 with SSL type.
POP3 - use port 995 and enable the option This server requires an encrypted connection (SSL), if available.
Outgoing server (SMTP) to use port 465 (or 587 if the submission port is enabled) with SSL or TLS type (depends on the server configuration).