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Timeouts can be used to improve sending performance. Some timeouts,
such as Timeout.resolver.retry, are new (V8.10 and
The discard delivery agent can be used to handle
bounced email (V8.9 and later).
The Local_check_rcpt rule can be used to create a
mail-reception black hole (V8.8 and later).
Buffered file I/O improves sending performance (V8.12 and later).
Multiple queues and queue groups aid queue performance (V8.12 and