g1smd - 7:57 pm on Oct 16, 2012 (gmt 0)
If you're only redirecting requests for index.php, then the rule cannot interfere with other .php requests.
If you were redirecting all .php requests to some sort of extensionless URL, then you could either exclude formmail requests or exclude POST requests.
There's more than one way to do this, and the right way to do it will differ for different sites. There's no one-size-fits-all cut-and-paste solution to be had.