The fix for this issue is available in the January 22, 2018, update for Outlook 2016 (KB4011123).

We're starting to see a few issues with the latest Office 2016 update for Microsoft Installer (.msi)-based version of Office. (These are typically used with volume licenses.)

If we have an e-mail in plain text with an attachment and attempt to forward that e-mail, Outlook strips the attachment. It started today after updates. The attachment is immediately stripped after hitting the forward button.

This behavior is the result of a code change in the MSI version, introduced in Security Update for Microsoft Outlook 2016 (KB4011626). Hidden attachments are deleted from plain text mails due to security concerns, as they can be visible when using certain protocols.

In this situation it is a bug, because visible attachments in plain text messages should not be deleted when you forward a message. Microsoft is investigating.

Until this is fixed, affected sites can either remove Security Update for Microsoft Outlook 2016 (KB4011626) and remove the update from WSUS (Windows Server Update Services) or instruct users to save the attachments and reattach them after clicking Forward. Tip: you can also drag the attachment from the original message to the Forward (if the forward is in a new window.)

The security update addresses the following non-security issues, so it's no surprise that it is the cause of the problem.

• When you select multiple attachments and try to cancel one of the attachments, all the other attachments are cancelled.
• Some attachments are not removed when you forward mails that contain inline images and you check the Read all mails as Plain Text check box.

Change Message Format

You can change the message format to HTML before you click Forward, but honestly, unless you can use a macro to do this, it's generally faster to save the attachment and reattach it.

1. Open the Message in a new window
2. Click Actions > Edit Message
3. Go to Format Text tab and change to HTML format
4. Save
5. Click Forward

You could use a macro in a run a script rule to change the messages to HTML as they arrive. Create a rule for the messages, add this script to the VB Editor.

If the option for a run a script rule is missing, see "Run-a-Script Rules Missing in Outlook" for the fix. For more information on using run a script rules, see "Outlook's Rules and Alerts: Run a Script"

Sub ChangeFormat(Item As Outlook.MailItem)
Item.BodyFormat = olFormatHTML
Item.Save
End Sub

See "How to use Outlook's VBA Editor" if you need help using the VBA editor.