AFPEx transmits the flight plan to the destination addresses immediately it's filed. It does not hold it until the day of flight. The software isn't that clever.
I was told by my local airport that they don't receive AFPEx filed flight plans until the day specified on the DOF/ parameter.
But now that you mention it, I will ask AFPEx to check this out.
Homebriefing.com have told me they delay VFR FPs as I described - this is not a bad thing as it avoids the most common source of lost VFR flight plans: the DOF/ parameter is not implemented anywhere other than a manual system by the receiving ATC (a nail in the wall, etc).