#1 Treat error as failed option

[Sebastien Boisvert]

Currently there doesn't seem to be a way to treat errors
as a failure; CheckUser returns an OK response.

Having this option would be useful in cases where the
server would not accept mail from the sending machine
because of a relay restriction (or similar), or another
possible error.

This would allow for some additional error checking on
the address instead of it being blindly accepted as good
when in reality it shouldn't be.