[exim] Local user enumeration through RCPT

Hi All

One of the servers we look after was recently "penetration tested" and they
could find very little wrong so they complained about silly things like it's
possible to see which users locally exist on the server through the answer
Exim provides to the RCPT command.

Any way of stopping this happening? I honestly can't see that it's such a
big problem myself and it looks like Exim is behaving just as it should.


Cheers
Phill