If there is a need for you to delete mailboxes, you may follow these steps:
Go to Portal then choose Hosted Exchange.
Select Manage, look for the email account to be deleted, or you may also use the search box. Click Edit on the right side to open the account details of the user.
Once you have the account overview opened, choose Delete this Account. A pop up window will show on your browser asking you to confirm the deletion. Position of the window will vary depending on the browser you are using. For Chrome, it appears on the top, for Firefox, it appears on the middle of the screen and grays out the rest of the page. Hit OK to proceed.
The next step is required only if you wish to purge the mailbox. It will be automatically purged after 72 hours and you should be able to either restore them, or purge them manually if you wish to. If you wish to do it manually, go to Undelete, choose the mailbox to purge, choose Action, then hit Delete.
You will get another confirmation window whichever action you selected. Just hit OK to proceed.