It seems like if you send an email with an attachment from Outlook 2013 or 2016 and your address is a gmail.com address people receiving the email may get an attachment called winmail.dat and it can't be opened.
Most often the receiver is using MacMail.
You can prevent some of the problems by doing these steps.
- Click File in Outlook.
- Select Options.
- Go to the Mail category.
- Make sure HTML or Plain Text is selected for Compose messages in this format: under Compose messages.
- Now make sure Convert to HTML format or Convert to Plain Text format is selected for When sending messages in Rich Text format to Internet recipients: under Message format.
- Click OK.
But if you have an Outlook.com email address and use Outlook Mail on the Web account, winmail.dat attachments may be sent to people in your address book no matter what your Outlook options are set to. This is an issue with Outlook and Outlook Mail on the Web, and you will need for Microsoft to update the applications to for it to be resolved. So use another mail client other that the web bases client.