Lets assume that you already have a working ssl for plesk URL

vi /etc/courier-imap/imapd-ssl

Edit

TLS_CERTFILE=/usr/share/imapd.pem

Now open

vi /etc/postfix/main.cf

Edit

smtpd_tls_cert_file = /etc/postfix/postfix_default.pem

Now Open the file

vi  /etc/courier-imap/pop3d-ssl

Edit

TLS_CERTFILE=/usr/share/pop3d.pem

Now copy

cp /usr/local/psa/admin/conf/httpsd.pem /usr/share/imapd.pem

and

 cp /usr/local/psa/admin/conf/httpsd.pem   /etc/postfix/postfix_default.pem

and

cp /usr/local/psa/admin/conf/httpsd.pem /usr/share/pop3d.pem

 

Now restart courier-imap and postfix

/etc/init.d/courier-imapd restart && /etc/init.d/postfix restart