Technical Details

SMTPRelay acts as a relay between your salesforce org and a third party mailserver. Salesforce allows you to use set a mailserver as the relaying mailserver, but this method does not support authentication.

SMTPRelay trusts the IP range of salesforce only. When it receives an email message from salesforce it checks to see with which mailserver it should be sent. The server then uses that mailserver with credentials and SSL/TLS to relay the message.