See the following eKB article --->
https://ironport.custhelp.com/app/answers/detail/a_id/695
By default, mail is queued for 72 hours (259200 seconds) OR 100 retry attempts before it bounces to the original sender.
This setting is configurable from the command line (CLI): type "bounceconfig" and edit the "default" settings. Also, you can modify this from the GUI interface by going to "Network > Bounce Profiles" and click on the Default profile.
Also, the queue could fill up if there is too much mail. However, if the system reaches its storage limit, it will soft bounce further attempts by other mail servers to deliver more messages. This ensures that no messages will get lost, as these mail servers will reattempt message delivery as well until the ESA accepts messages again.
Note: If you plan to shut down your internal mail server for maintenance for a longer period (more than a couple hours), best practice is to suspend the incoming listeners on your Email Security Appliances as well (CLI: suspendlisteners). As mentioned before, in this case any connection attempts will be soft bounced, and retried later. This way, you leave the task of storing the messages to the sending mail server, which will prevent the mail queue on your email appliances filling up quickly. No messages will be lost however, once you got your internal mail server back into service, also resume the listeners on your Email Security Appliances (CLI: resume), to allow delivery from remote hosts again.
I hope this helps!
-Robert
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