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

Introduction

• winmail.dat is the container used by Outlook/Exchange when a message is sent in RTF/TNEF format.
  • It encapsulates the formatting and attachments.
• Many non-Microsoft software/email clients cannot interpret TNEF:
  • the formatting disappears, and the attachments become a single winmail.dat file.

Resolving the issue (actions to request from the sender)

The most effective solution is to have the sender modify their Outlook configuration.

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
  Compose: Compose messages in the following format: choose HTML (or Plain Text).
  Message format: in When sending rich text messages to Internet recipients, select Convert to HTML (or Convert to plain text).
  On a case-by-case basis (single email): in the message window, Format Text tab > choose HTML.

• New Outlook for Windows (application based on Outlook on the web)
  Menu: Settings (gear icon) > Mail > Compose and reply
  Message format: Compose messages in: choose HTML (or Plain text).
  This client does not offer 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 > Writing > check/uncheck Write messages in HTML by default.
  For a single message: in the writing window, Options tab > toggle HTML or Plain Text.

Note: 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 (e.g., yourself)

• Classic Windows Outlook: open the Contact card > double-click the email address > Outlook Properties > Internet format: select Send in plain text only (or Let Outlook decide if the administrator is already forcing the conversion).

• Clear the auto-complete (it may “memorize” RTF for a recipient): File > Options > Mail > Send messages > Clear the auto-complete list, then retype the address manually the next time you send a message.

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

If this doesn't work: use another email client to send the invitation (e.g. Thunderbird), or check that calendar invitations are sent in iCalendar format (File > Options > Calendar > enable Use the iCalendar format for external recipients).

If you do not wish to contact the sender again

You can extract the content of winmail.dat using 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 permanent solution. Furthermore, confidentiality and security are in no way guaranteed during these operations.