Assume that you use the Set-OwaVirtualDirectory cmdlet together with setting the FilterWebBeaconsAndHtmlForms property to ForceFilter to block all web beacon and HTML form contents (externally linked images) in individual email messages in a Microsoft Exchange Server 2013 environment. When you forward or reply an email message that contains these web beacons in Microsoft Outlook Web App, the blocked images are still downloaded.
This issue occurs because the value of the FilterWebBeaconsAndHtmlForms property is not checked when email messages are forwarded or replied.
To resolve this issue, install Cumulative Update 9 for Exchange Server 2013.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
For more information about web beacon and HTML Form Filtering in Outlook Web App, see Understanding Web Beacon and HTML Form Filtering in Outlook Web App.