we have outlook 2013 and exchange 2013 inhouse.
when one of our remote users send an email to his salesforce (they have private email accounts, AOL, AT&T, so on) several of them get the winmail.dat instead of the attachment.
I told him how to verify he was sending the email in HTML format, and he said he checked and it was HTML, but when he sent the email, several of the salesforce received the winmail.dat file.
is that possible when the email is in HTML format?