Certificate for SMTP over SSL is located in file:

/var/qmail/control/servercert.pem

For IMAP4 and POP3 over SSL the following certificate files are used accordingly:

/usr/share/courier-imap/imapd.pem

 /usr/share/courier-imap/pop3d.pem

 By default these are self-signed certificates for 'plesk' name which are generated during SWsoft Plesk installation.

It is normal, but when customer want to add his own certificate he must change that file manually.

It can be done  by adding certificate keys from Plesk control panel: Home > Settings > SSL Certificates

If you change sertificate for courier-imap for example, the original Plesk certificate is saved under name /usr/share/courier-imap/imapd.pem.original

After that, restart courier-imap service.

More informations can be found on:

http://kb.parallels.com/en/1062