Barry Warsaw writes:
But it would have to be subject to the same bounce rules as any other
auto-response which could be used as a spam vector, e.g. limit the
number of bounces per time period and don't include the entire
original message in the bounce
But that prevents detecting a prematurely sent message, which is presumably a common use case for genuine collisions.
I just don't think bouncing back is going to be very useful; either you don't give the user the information he needs to figure out what happened, or you give the spammers a vector.