1,Get a free SSL-Cert or cheap one
StartSSL™ Comparison Chart
Is it okay to use the FREE SSL Certificate offered by: Instant SSL by comodo? or Is StartCom better? [closed]
You Can get less than $10/year fro cert from namecheap
#cat ssl.crt sub.class1.client.ca.pem > /etc/ssl/certs/dovecot.pem
# openssl rsa -in ssl.key -text > ssl-private.pem
# cat ssl-private.pem > /etc/ssl/private/dovecot.pem
#chmod 644 /etc/ssl/private/dovecot.pem
#chmod 444 /etc/ssl/certs/dovecot.pem
3,It’s OK to use a Selfsigned SSL for mail server
It is definitely worth enabling SMTP TLS if you can, whether or not you pay for a third-party-signed certificate.
If you are concernced about your mail being marked as spam, consider setting up SPF and DKIM.