When attachments and calendar invites appear as winmail.dat
When an incoming e-mail contains a single attachment named
winmail.dat, a Windows user has misconfigured their Outlook client. They are probably trying to send you an attachment or calendar invite, which are now compressed into the
winmail.dat attachment using a proprietary format. Outlook will also attach a
winmail.dat if such an user wrote a formatted e-mail.
If possible, ask the sender to fix their Outlook configuration:
Gmail can often decode
winmail.dat files and show the contained events and attachments.
If you are not using Gmail, consider forwarding the e-mail to a temporary Gmail account for this.
There is an Ubuntu package called
winmail.dat files, but it never worked for me. It just exits silently without doing anything.
Install a Windows VM with Outlook.