SRS - rewrite sender email - enable or disable
With SRS on a Mail Server, you can select to enable or disable SRS, which is the ability for the mail server to re-write the sender's email address. This is generally done so that forwarded messages pass SPF checks.
In theory, this sounds like a great idea. However, gmail on their "Best practices for forwarding mail to Gmail" help article, specifically says NOT to do this:
Details here on gmail comment: https://support.google.com/mail/answer/175365?hl=en
In SmarterMail, it is possible to set this on a per domain basis in case this is something you want to try to resolve SPF check issues.