Some users are discovering that Outlook sends winmail.dat files attached to messages when sending to contacts in their address book. Only Outlook on Windows desktop (all versions) is affected; messages sent from Outlook on the web or the Outlook app do not have the rogue winmail.dat file.
The problem: When sending email to any contacts from your address book in Outlook, some recipients receive a winmail.dat file attachment that shows up in some clients. Outlook should decode the winmail.dat file and correctly display any attachments, however, if not, the attachment icon may be displayed but no attachment is shown.
Gmail & Yahoo web mail should decode the winmail.dat file and show the attachment but HTML formatting will be lost. The Internet header will mention the winmail.dat file:
Content-Disposition: attachment; filename="winmail.dat"
Content-Type: application/ms-tnef; name="winmail.dat"
The keywords are "address book". If you select a contact and click the Email command on the ribbon or right-click and choose Create > Email from the context menu the message does not have the winmail.dat attachment.
As has always been the case, the winmail.dat file is more of a confusing nuisance than anything. The recipients receive an attachment and don't know what to do with it. (Correct answer: ignore it.) Note: Gmail and Yahoo recipients will lose HTML formatting but attachments will be accessible.
The usual fixes (listed at Outlook is Sending Winmail.dat Attachments) don't seem to work for most people or are only temporary and the problem returns. This is likely because the address book is the problem, not the contacts specifically.