Just to add a concrete number to it - in the default config mails which could not be sent due to a temporary error (network timeout, or the downstream server replying with a 4xx code) are kept for 5 days - see http://www.postfix.org/postconf.5.html#maximal_queue_lifetime
Just to add a concrete number to it - in the default config mails which could not be sent due to a temporary error (network timeout, or the downstream server replying with a 4xx code) are kept for 5 days - see http://www.postfix.org/postconf.5.html#maximal_queue_lifetime