The "Initial destination concurrency" option specifies the initial per-destination concurrency level for parallel delivery to the same destination. This limit applies to delivery via SMTP, via the local delivery agent and via the pipe mailer.
With concurrency of 1, one bad message is enough to block all mail to a site. A concurrency of 2 seems a reasonable choice. This option sets the "initial_destination_concurrency" postfix variable.
The "Maximal backoff time" option specifies the maximal time in seconds between attempts to deliver a deferred message. This option sets the "maximal_backoff_time" postfix variable.
The "Maximal queue lifetime" option specifies the maximal time in days a message is queued before it is sent back as undeliverable. This option sets the "maximal_queue_lifetime" postfix variable.
The "Minimal backoff time" option specifies the minimal time in seconds between attempts to deliver a deferred message. This parameter also limits the time an unreachable destination is kept in the short-term, in-memory destination status cache. This option sets the "minimal_backoff_time" postfix variable.
The "Queue run delay" option specifies the time in seconds between deferred queue scans by the queue manager. This option sets the "queue_run_delay" postfix variable.
The "Defer transports" option specifies the names of transports that should not be delivered to unless someone issues "sendmail -q" or equivalent. Specify zero or more names of mail delivery transports names that appear in the first field of master.cf). This option sets the "defer_transports" postfix variable.