Hello Jeffrey, normally this points at messages being sent in RTF format within Outlook. When configured this way Outlook will send attachments in winmail.dat format. I believe you can simply switch the compose format to something like plain text and that should clear up those dat files. Hope that helps!

my new client having a same issue with his apple mail, he can not open his Winmail.dat attachments, is there anything can we do on recipient side instead of sender?

Unfortunately there isn't anything the receiver can do to resolve this Kamlesh, as the problem is in how it was sent. I believe the same setting apply though, so you can try disabling the Rich Text Format to see if this helps. If you need assistance tracking this down please don't hesitate to submit a ticket with our support team.