Section 6.2
Timeouts can be used to improve sending performance. Some timeouts, such as Timeout.resolver. retry , are new (V8.10 and later).
Section 6.3.2
The discard delivery agent can be used to handle bounced email (V8.9 and later).
Section 6.3.3
The Local_check_rcpt rule can be used to create a mail- reception black hole (V8.8 and later).
Section 6.4
Buffered file I/O improves sending performance (V8.12 and later).
Section 6.5
Multiple queues and queue groups aid queue performance (V8.12 and later).