I was contacted by a few users saying they can "FLAG" a message using Yahoo, and Gmail, but not GroupWise.
I found an Apple tech questions showing a Fix for gmail by changing from Activesync to imap and another doing a change on the username to use the email format.

Can anyone verify if you can Open a message and select the Flag icon.
There "Should" be options for Flag, Mark as Unread, and Cancel.
I only have Mark as Unread, and Cancel.