CP is, in reality, a fairly infrequent crime. It is also a crime, meaning that someone human should definitely pay attention to it - unless you propose robotization of sentencing as well.

If your automatic detection is tuned well enough not to produce shittons of false positives, you should have enough people to review suspected cases. Real positives ought to be escalated to actual police.

(Also, actual police will not be happy about an automatic mechanism that locks out real CP producers out of their mailboxes, thus alerting them to the fact that there is something going on, and possibly giving them time and opportunity to destroy some incriminating evidence. There is a reason why suspected criminals are staked out carefully, then suddenly raided.)