To securely exchange mail data with Plesk server, you may need to
install custom SSL/TLS certificates on the Plesk server. Specifically,
SSL/TLS certificates can be installed for the Qmail mail transfer agent
and the Courier-IMAP mail server that supports the IMAP and POP3
protocols.
To install custom SSL/TLS certificates, you need to download the
certificates to the Plesk server and then replace the installed default
SSL/TLS certificates for Qmail and Courier-IMAP servers with the
downloaded custom certificates.
This section describes procedures for installing custom SSL/TLS
certificates for Qmail and Courier-IMAP servers.