In my previous post Custom Domain E-mails With Postfix And Gmail: The Missing Tutorial, we set up a Postfix mail server on a custom domain that integrates seamlessly with Gmail.

However, the tutorial skipped two important security standards that will help prevent e-mails routed through our server from being marked as spam: DKIM and SRS. This article will show you how to add support for DKIM and SRS to a Postfix server. //

DKIM, short for DomainKeys Identified Mail, is a mechanism for

1. A sender e-mail program to sign an outgoing e-mail message, and
2. A recipient e-mail program to verify said signature.

SRS, short for Sender Rewriting Scheme, is a standard for including forwarding / relay information in a forwarded / relayed e-mail message.