When forwarding a mail that I already sentl to a new group of recipients (1 common exchange user, 3 outside mail adresses), Outlook 2013 graphically marks the new message as an appointment request. The Exchange server returns the mail with an error message:
#550 5.6.0 M2MCVT.StorageError; storage error in content conversion ##
The only workaround is to temporally disable GPGol.