Hi,
Here is what I made to add a SSL for my mail server:
Keep in mind: The location for generated csr and key are in /etc/pki/tls/certs (for csr) and /etc/pki/tls/private (for key)
1. I bought a SSL certificate Comodo Positive SSL (it was the cheapest, any certificate would do this trick.) for the hostname (in your example: mail.domain.example).
2. Upload the generated certificates (yourdomainname.crt and yourdomainname.ca-bundle) on /etc/pki/tls/.
3. Combine the uploaded files into one:
cat /etc/pki/tls/yourdomainname.crt /etc/pki/tls/yourdomainname.ca-bundle >> /etc/pki/tls/certificate.crt
4. Open /etc/postfix/main.cf and edit the lines:
smtpd_tls_cert_file=/etc/pki/tls/certificate.crt
smtpd_tls_key_file=/etc/pki/tls/private/yourdomainname.key
5. Delete from /etc/postfix/main.cf the lines with CAcert file, I had 2 of them in the file.
6. Open /etc/dovecot/dovecot.conf and edit the lines:
ssl_cert = </etc/pki/tls/certificate.crt
ssl_key = </etc/pki/tls/private/yourdomainname.key
7. Save and close the files, restart mail services, and your mail server SSL is working 100%.
Just be sure to enter the right path for your files! Verify your paths first.