Decides if a recipient should be accepted. The decision can be a final
positive, a final negative, or a neutral answer. This function is not
called if a previous filter has already accepted or rejected the
recipient. In case of a neutral answer, other filters will decide. If all
filters return the neutral answer, then the recipient will be accepted if
a destination is assigned to it and the assigned destination is not an
UnknownRecipientDestination
; otherwise it will be rejected as an
unknown user.