You get reports that external recipients receive attachments as winmail.dat attachment ( or sometimes ATTxxxxx ). The cause is the format that has been used to send the mail. This behavior occurs in most cases when an email with attachment has been sent from an Outlook Client and the external recipient does not use Outlook.
However, to solve this issue on client side, the external recipients having this problem, should be present in your Contacts folder.
> Open the contact and double click it’s email address
> Expand the right icon and choose Outlook properties
> Click OK and resend the mail with attachment to this contact.
It does not help to specify in the settings to send Emails in HTML format. You need to force it on the recipients.
See a server side solution here :