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Postfix does not have jitter for deferred mails and scans the queue periodically every queue_run_delay (<https://www.postfix.org/postconf.5.html#queue_run_delay>). As a result it is likely to try delivering many deferred messages at the same time. Normally the number of outgoing connections should be low even with unreachable destinations, but after the server downtime or if admin flushes the queue manually it is possible that a lot of messages to the same unreachable destination expire at once and are moved from "deferred" into the "active" queue. Trying to deliver them all at once may make the server run out of memory by starting many LMTP clients. Limiting the number of LMTP processes turns OOM problem into head of line blocking problem. Messages sent to reachable destinations will be delayed as well, but at least deferred messages will get distributed over time. In this case "active" queue may grow (up to qmgr_message_active_limit defaulting to 20000), but then admin may notice the problem and solve it e.g. by making the destinations reachable or setting up a transport map to route messages for known dead servers into discard transport. Eventually the problem should be solved by filtermail-transport quickly returning temporary errors for destinations which already have many messages queued, then we can reduce "maxproc" further.