If you receive email from people using older versions of Microsoft Outlook, you may see winmail.dat attachments on messages. These attachments contain formatting information in Microsoft's proprietary TNEF format, but they cannot be read by many other email clients including Mailspring.
The easiest way to handle them is to get your correspondent to not send you 'rich text' format emails from Microsoft Outlook:
Many IT departments also configure Microsoft Exchange not to send winmail.dat files outside their corporate firewall, which is considered a best practice by many Exchange administrators.
If you receive winmail.dat attachments in Mailspring and would like to open them, you can find utilities for Mac, Linux and Windows that allow you to decode the proprietary TNEF format.