Resolve an issue with an unreadable winmail.dat attachment

This guide concerns files attached by your correspondent that appear in the winmail.dat format when they arrive at your Infomaniak-managed email address.

These files are in no way dependent on the incoming or outgoing mail provider. It is a Microsoft-specific issue, as indicated on their support page or this page (in English).

What is winmail.dat?

The issue with "winmail.dat" attachments is quite common and can occur when sending emails from Microsoft software/clients such as Outlook. The "winmail.dat" format is used to encapsulate formatting information, attachments, and contacts within the message. However, this can cause problems when recipients use other software/clients that cannot correctly interpret this format.

Resolving the Attachment Reading Issue

Ask the sender to:

1. Resend the message without using the "winmail.dat" format, by using a standard email format such as HTML or plain text when sending messages, and/or disabling RTF formatting in Outlook settings before sending the email:

2. Disable sending in RTF format for recipients who are not using Microsoft software/clients. To do this, they need to open your contact record in their contact list, go to the "Email Address" tab, and select "Send in plain text only"
3. Compress the attachment into a standard zip file before sending it
4. Use another software to send their message, such as Thunderbird, for example

If you do not wish to contact them again:

1. Use free websites and online tools to decrypt the content and convert the "winmail.dat" file into a more or less readable format