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Messages in the "maildrop", "hold", "incoming" and "deferred" queues (see below) do not occupy memory; they are safely stored on disk waiting for their turn to be processed. The envelope information for messages in the "active" queue is managed in memory, allowing the queue manager to do global scheduling, allocating available delivery agent processes to an appropriate message in the active queue.
Within the active queue, (multi-recipient) messages are broken up into groups of recipients that share the same transport/nexthop combination; the group size is capped by the transport's recipient concurrency limit.
Multiple recipient groups (from one or more messages) are queued for delivery grouped by transport/nexthop combination. The destination concurrency limit for the transports caps the number of simultaneous delivery attempts for each nexthop. Transports with a recipient concurrency limit of 1 are special: these are grouped by the actual recipient address rather than the nexthop, yielding per-recipient concurrency limits rather than per-domain concurrency limits. Per-recipient limits are appropriate when performing final delivery to mailboxes rather than when relaying to a remote server.