mfcmapi not changing properties

A user is running Exchange 2013 with Outlook 2016 and we've found that a large number of incoming messages have being marked as Drafts. This means they cannot be replied to and it is causing a lot of grief to the user.

I found suggestions on using MFCMAPI to modify the messages to change the PR_MESSAGE_FLAGS but when I do i get the following error.

Problem array:
Index: 0x00000000, ulPropTag: Tag: 0x0E070003
Type: PT_LONG
Property Name: PR_MESSAGE_FLAGS
Other Names: PidTagMessageFlags, ptagMessageFlags
DASL: http://schemas.microsoft.com/mapi/proptag/0x0E070003, scode: 0x8004011A = MAPI_E_COMPUTED

In file UI\Dialogs\Editors\PropertyEditor.cpp
On line 606

I am running 64bit Office and the 64bit version of MFCMAPI (MFCMapi.exe.x64.18.0.18058.06)

Any ideas?