If you’re a Mac user, chances are that you’ve probably received some emails with a winmail.dat or ATT0001.dat attachments. Sometimes there should be an attachment in the mail, but you can’t open it because the only thing attached is the winmail.dat file. There are a few scenarios that can cause this, and when searching the web, there’s no one solution, only workarounds.
Basically, winmail.dat is a proprietary file used by Microsoft Outlook and Microsoft Exchange when it sends out rich text files. Because of this, the Mac Mail app doesn’t know how to decode this file to open it. Luckily, there are a few things you can do to open these files.
You can download the app from here. This is basically a decoder app that you can save on your Mac and run it as needed or keep in your dock. When downloading it, just save it into your Applications folder. You can read more about it from the download link above.
This is a handy website that allows you to upload your winmail.dat file if it is under 5MB. After uploading, the site will convert the file and let you see its contents.
Suggestions To The Sender
There are a few things you can tell the email sender that might help prevent the winmail.dat file from coming over in the future. First, in their Outlook settings, they can configure it to compose emails as HTML or Plain Text instead of Rich Text. You can read more from Microsoft here.
Another suggestion is to have the sender delete your contact information and re-create it.
If you have any other suggestions that have worked for you, please post them in the comments.