Mail default SSL certificate

Hi,

I enabled mail for all clients on my Plesk 12.5 server.

When an mailadress is added to a mail client, there is a popup each time for the SSL certificate.
This because the certificate is not trusted and the user needs to connect/trust the certificate mannualy.

I use Postfix for mail and Dovecot as IMAP/POP3

Is there a way to fix this, without purchasing a SSL for each domain?

I use mail like:

webmail.domain.com
mail.domain.com
smtp.domain.com

For example is it possible to create my...

Mail default SSL certificate