Let's say you want to send an email from john.doe@hotmail.com to jane.doe@example.com, where jane.doe@example.com is an account on your Postfix Dovecot email server. Hotmail is going to need some way of determining the IP address of example.com. You will need to configure your domain name MX (mail) records to point to the public IP address being used by your Postfix Dovecot email server. For example, in this image, example.com public IP address is 12.34.56.78, and the MX record then points to mail.example.com.

http://mxtoolbox.com can be used to verify that your MX record configuration is working properly on the Internet.

Open port 25 (SMTP) and port 587 (SMTPs) in your router and firewalls, and ensure your router is configured to point SMTP / SMTPs traffic to the privte IP address of your Postfix Dovecot email server.

You can now send a test email from john.doe@hotmail.com to jane.doe@example.com, and the email should be delivered to jane.doe@example.com.