Where held-back mail goes, and why their names matter important
Mail held back by a rate limit, a flood pattern, a pause or greylisting is moved into a queue created on the spot and named after what is holding it.
In detail
Normal traffic keeps flowing while held traffic waits elsewhere — that is the point of the mechanism, and it is why one slow domain does not block the queue behind it. The queues are named for what they hold: to: domain.com (id), or from: [address] to: domain.com (id) when the hold is specific to a sending address, with an id appended so names cannot collide. For us the naming is the valuable part. MailerQ's sending queue is one flat list with no per-domain breakdown; the temporary queues are the only place where the engine says out loud which domains are currently in trouble and how much mail is stuck behind each. A monitoring panel that lists them is showing something no counter exposes. Not yet observed on our stand: no throttle has held anything back there. The mechanism and the naming pattern are therefore recorded from documentation and marked unverified until a limit is configured and mail is pushed against it.