Why the old Postfix queue manager was replaced

The old Postfix scheduler had several limitations due to unfortunate choices in its design.

1. Round-robin selection by destination for mail that is delivered via the same message delivery transport. The round-robin strategy was chosen with the intention to prevent a single (destination) site from using up too many mail delivery resources. However, that strategy penalized inbound mail on bi-directional gateways. The poor suffering inbound destination would be selected only 1/number-of-destinations of the time, even when it had more mail than other destinations, and thus mail could be delayed.

   Victor Duchovni found a workaround: use different message delivery transports, and thus avoid the starvation problem. The Patrik Rak scheduler solves this problem by using FIFO selection.

2. A second limitation of the old Postfix scheduler was that delivery of bulk mail would block all other deliveries, causing large delays. Patrik Rak's scheduler allows mail with fewer recipients to slip past bulk mail in an elegant manner.