When an email on an IMAP e-mail server is marked for deletion, the email is displayed in strikethrough text in the inbox listing. However, the item is not removed from the IMAP mail server until a purge command is used. This functionality differs from POP3 accounts, which move an email to the Trash (or Deleted Items) folder when deleted.
As items are not automatically removed from the mailserver when marked for deletion, you may find that your quotas can quickly be exceeded. There are two methods that can be used to removed messages from your IMAP mailserver: manual or automatic purging.
To permanently delete the items marked for deletion, do the following:
You can also configure Outlook to automatically purge items when you switch between folders on your IMAP account.