Loop detection

Before every send the guard reads this key's mail from the last 60 minutes. A candidate that would be the 4th near-identical message to the same recipient is held. Near-identical means subject and body similarity of at least 85%.

On a key set to full, and on a marketing send from a key with no setting, it also holds the next send to an inbox that a window of at least 20 sends has collapsed onto, once that window has fewer than 3 effective recipients. Mail to anyone else still sends. A new key is repetition-only. The hold is an approval queue entry, answered as approval_required.