For Google Apps.
For Office 365.
Why do I receive emails with Winmail.dat attached?
You may receive the file of “Winmail.dat” attached e-mail if the e-mail was written by the rich text format and sent from Microsoft Office Outlook.
・Microsoft support page
If the receiving-side client is a Microsoft e-mail client, "Winmail.dat" will be treated as a file for displaying rich text format.
If the receiving-side client is not a Microsoft e-mail client, in spite of sender does not attach a file, phenomenon that receives the file of "Winmail.dat" attached e-mail may occur.
The following is a flow for outgoing e-mail applies the above-mentioned case.
1.Sender sends e-mail in Rich Text Format（Attach the “Winmail.dat” unintentionally）
2.By filtering, it detects the file of 1. as an attachment, and it encrypts and sends a notification e-mail password
3.Recipient receives e-mail（Receives the encryption-PW-notification e-mail too）
a) If you are using Microsoft Client
⇒ Does not recognize as a Winmail.dat attachment＝File is not attached, but receive a password notification
b) If you are using the client which is not a Microsoft
⇒ Winmail.dat will be recognized as an attachment＝File is attached, and receive a password notification
If the password notification mail has arrived in spite of not attach a file, it depends on mail clients which sender is using and type of mail created.
However, all cases do not apply to any of above, please attach the relevant message source and contact our support team.
In order to solve the problem of "Winmail.dat", you are possible to utilize the settings for "Extension for not encrypted mail".
Register the "dat" as "Extension for not encrypted mail", "dat" file makes it possible to prevent unintended encryption, and the ZIP encryption is invalid.
*Please note that the ZIP encryption is invalid except “Winmail.dat”