You can get to the mail box in a few different ways, the following is one of them, Go to Settings -> Email Configuration
Click on Mailboxes
Select the mailbox, Click Approve and then click on Test and Enable
Once the test and enable completed, you will be able to see if it was successful or not in the test status columns as shown below
In the approach i mentioned above, you can bulk approve and enable multiple mailboxes.
You can also open the mailbox and do the same as shown below
Once you approved and clicked on Test and Enable, you can check the status under Alert tab. it might take a few minutes to get the results in the alert area as its an async task that tests the connection.