How long does it take to turn a feature into a bug?

Looking at the current spam attack at our BTS I wonder how long it'll take 'till the feature that our BTS is controllable via mail becomes a pain in the ass because of the spam.

I'm sure we have some spamfilter and some brave people controlling it, since in the last time it was relative quiet and spam free. But the current attack makes me think if we couldn't raise the barrier for spam somehow. Not sure how to achieve that though.