You can use orokos.com for PBEM or PBP.
Otherwise, you can do a couple things. If your heart isn't set on email or a specific forum as the mechanism, you can play over your own subforum using a site like rpg.net or even Slack which have built in dice rollers.
Alternately, for real time (both you and the player are online at the same time), you can use Roll20. For PBEM, this is not likely to be the norm, but it is an option.
I should note that all will require an account, so as a one time only thing it might be sub-optimal. In that case, you can use Stack's dice roller, assuming the other person has a Stack Exchange account already as well.