I don't know how laws work here but I can't imagine having adequate "proof of service" fully insulates you from all possible claims of non-receipt, especially electronically? Like what if the recipient was in a coma or on active duty in the middle of a war zone or something? There have got to be exceptions here to handle some cases of non-receipt despite proof of delivery, so the question of whether spam classification might be one such exception doesn't seem automatically invalid.