This guide explains why some attachments sent from Microsoft Outlook arrive in winmail.dat format on your email address hosted by Infomaniak, and how to permanently resolve the issue on the sender's side (where it originates).

Preamble

• winmail.dat is the container used by Outlook/Exchange when a message is sent in RTF / TNEF.
  • It encapsulates the formatting and attachments.
• Many non-Microsoft email clients do not know how to interpret TNEF:
  • The formatting disappears and the attachments become a single winmail.dat file.

Solving the problem (actions to request from the sender)

The most effective solution is to modify the Outlook configuration on the sender's side.

Disable RTF/TNEF and resend the message in HTML or plain text

• Classic Outlook for Windows (Microsoft 365/2021/2019, Win32 application)
  Menu: File > Options > Mail
  Composition: Compose messages in this format: choose HTML (or Plain Text).
  Message format: in When sending messages in Rich Text format to Internet recipients, select Convert to HTML (or Convert to Plain Text).
  Case by case (a single email): in the message window, tab Text Format > choose HTML.

• New Outlook for Windows (Outlook on the Web-based application)
  Menu: Settings (gear icon) > Mail > Compose and reply
  Message format: Compose messages in: choose HTML (or Plain Text).
  This client does not expose an RTF option; sending in HTML/text avoids winmail.dat.

• Outlook on the Web (Exchange Online/Outlook.com)
  Menu: Settings > Mail > Compose and reply > Message format > choose HTML (or Plain Text).

• Outlook for Mac
  Menu: Outlook > Preferences > Composition > check/uncheck Compose messages in HTML by default.
  For a single message: in the composition window, tab Options > switch to HTML or Plain Text.

Notes: if the sender has attached files, they will be readable by all recipients as long as the message is sent in HTML/text. Zipping the attachments (.zip) does not prevent Outlook from encapsulating the message if RTF/TNEF is still active.

Disable RTF for specific recipients (you, for example)

• Classic Outlook Windows: open the Contact card > double-click the email address > Outlook Properties > Internet Format: select Send as plain text only (or Let Outlook decide if the admin already forces the conversion).

• Clear the auto-completion (it may “remember” RTF for a recipient): File > Options > Mail > Send messages > Empty the Auto-Complete List, then retype the address manually for the next send.

• Microsoft 365 administrators (at the sender's): in the Exchange Admin Center: Mail flow > Remote domains > Default > Rich Text format: choose Never. In PowerShell: Set-RemoteDomain Default -TNEFEnabled $false.

Failing that: use another client to send (e.g., Thunderbird), or check that Calendar invitations are sent in iCalendar (File > Options > Calendar > enable Use iCalendar format for external recipients).

If you do not wish to recontact the sender

You can extract the content of winmail.dat with TNEF decoding tools (online or desktop applications). Search for “open winmail.dat” and choose a reputable tool for your system (Windows, macOS, Linux). This usually restores the attachments, but it is not a lasting fix. Moreover, privacy and security are in no way guaranteed during these operations.