Microsoft provides the following guidance for purging email messages in Outlook;

Automatically purge the deleted items

You can configure Microsoft Outlook so that when you switch between folders, any items you have marked for deletion are purged from the IMAP mail server.

1. Click a folder in your IMAP e-mail account.
2. On the Edit menu, point to Purge Deleted Messages.
3. Click Purge Options.

The Internet E-mail Settings dialog box opens.

4. On the General tab, under Purge Options, select the Purge items when switching folders while online check box.

Note You must be working online with the mail server for this feature to work.

Purge IMAP items marked for deletion – Microsoft Office