Non-sending domain flag

Some domains exist only to receive mail, or are parked/unused entirely — they should never appear as a sender in anyone's DMARC reports. Flagging non_sending on the domain page tells this tool that assumption, so R10 can flag any observed traffic as spoofing rather than silently ignoring it as ordinary activity.

A non-sending domain is also usually a strong p=reject candidate — there's no legitimate traffic that enforcement could break.