EnterraHost
Quarantine holds mail the filter decided not to deliver. It is not deleted mail and it is not a second inbox. It is a holding area, and a message in it can almost always be delivered after the fact.
Finding the message
The quarantine interface lists what is being held, with the sender, subject and the reason it was stopped. Searching by sender is the fastest route when you know who wrote to you and the message never appeared.
Quarantine is not permanent. Messages are held for a limited period and then discarded, so a message you need is worth releasing the day you notice it rather than at the end of the week.
Delivering it
Releasing a message sends it on to your mail server as though it had just arrived, so it turns up in the recipient's inbox normally. There is no separate place to look for it afterwards.
Stopping it happening again
Releasing one message fixes one message. If the same sender keeps being held, the fix is to allow them, which is the durable version of the same action. Whitelisting a sender covers it, and it is the better answer for anything regular such as an invoice system or a mailing list.
Worth knowing before you allow a sender wholesale. An allow entry is a promise to accept everything from that address, including anything a compromised account sends later. Allow the specific sender rather than the whole domain where you can, and read reporting a false positive for the option that improves the filter rather than working around it.
What quarantine is not
It is not a backup of your mail. Messages that were delivered and later deleted are not in quarantine, and neither is anything that arrived while your MX records still pointed straight at your server. If you need recoverable history rather than a holding area, that is archiving, which keeps a copy of everything.