[mmorse]

maximal_queue_lifetime (default: 5d) The maximal time a message is queued before it is sent back as undeliverable.
Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks). The default time unit is d (days).
Specify 0 when mail delivery should be tried only once.

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You essentially want maximal_queue_lifetime in postfix's main.conf
However, you cannot edit main.conf directly-it will overwrite on every startup. (essentially you will never get a change to apply it via a postfix refresh.)

so:

>su zimbra
and read up on maximal_queue_lifetime via:
>man 5 postconf (scroll down/search for the maximal_queue_lifetime section)
to learn how postconf works also read:
>man postconf

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also kinda relevant to you/anyone else searching this thread later:

queue_run_delay (default: 300s) The time between deferred queue scans by the queue manager; prior to Postfix 2.4 the default value was 1000s.
This parameter should be set less than or equal to $minimal_backoff_time.
Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks). The default time unit is s (seconds).

minimal_backoff_time (default: 300s) The minimal time between attempts to deliver a deferred message; prior to Postfix 2.4 the default value was 1000s.
This parameter also limits the time an unreachable destination is kept in the short-term, in-memory, destination status cache.
This parameter should be set greater than or equal to $queue_run_delay.
Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks). The default time unit is s (seconds).

maximal_backoff_time (default: 4000s) The maximal time between attempts to deliver a deferred message.
This parameter should be set to a value greater than or equal to $minimal_backoff_time.
Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks). The default time unit is s (seconds).