This link explains 'winmail.dat': * https://support.mozilla.org/en-US/kb/what-winmaildat-attachment Basically, the sender is using a Microsoft product - probably Outlook email client. They are wrongly assuming that all their recipients are using Microsoft Outlook. They may not even be aware that they need to sort out this setting before sending emails. Advice: send them the following link and tell them to switch TNEF off. * https://support.microsoft.com/en-us/help/290809/how-e-mail-message-formats-affect-internet-e-mail-messages-in-outlook