During your testing, it might be useful to be able to empty a mailbox to perform several tests.
This article explains how to empty a mailbox in Office365 or Exchange using PowerShell.

How to delete mailbox content in Office 365 or Exchange

There is a useful command in PowerShell (search-Mailbox -DeleteContent)
This cmdLet is available by default in Exchange OnPremises.
However, an additional step is necessary in Office365 to enable this cmdLet: By default, it’s not exposed because the “Mailbox Import Export” role is not set.

Enable “Mailbox Import Export” role

  • Connect to Office365 Portal.
  • Select Admin -> Exchange.
  • Click on Permissions.

Office 365 roles

  • Double-click on Organization Management.

Organization Management

  • Under Roles, click on the plus sign (+).

Mailbox import

  • Add the Mailbox Import/Export role.
  • Then ensure that your account is a member of the Organization Management.

Delete the content of a mailbox

Run: Search-Mailbox -id -DeleteContent -force
example: Search-Mailbox -id vuser1 -DeleteContent -force

This will delete all content (mails, contacts, calendars).
To verify, you can issue the command get-mailboxfolderstatistics | select FolderPath,ItemsInFolder

Note: You will notice that it does not delete personal folders.
Deleting folders needs an additional step.

Check this thread: it provides a small script for deleting folders:
http://social.technet.microsoft.com/Forums/windowsserver/en-US/bc07df23-64f1-4732-a399-76fd39b1595e/powershell-exchange-2010-delete-folder-in-mailbox-or-delete-mail-in-certain-folder?forum=winserverpowershell