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Help · Outlook / Exchange

Customers receive a winmail.dat attachment instead of your file

winmail.dat (142 KB)
The recipient sees a single unreadable attachment; your PDF, calendar invite or formatting is inside it.

What's actually happening

Outlook sent the message in TNEF (Transport Neutral Encapsulation Format), a Microsoft-only rich-text wrapper. Anything that isn't Outlook — Gmail, Apple Mail, most phones — can't unpack it, so the whole message body and attachments arrive as winmail.dat.

Fix it for the whole organisation (best)

Set-RemoteDomain -Identity Default -TNEFEnabled $false

Run in Exchange Online PowerShell. This tells Exchange never to send TNEF outside your organisation. It's the one-line permanent fix.

Fix it for one sender

  1. In Outlook: File → Options → Mail, under Compose messages set the format to HTML (not Rich Text).

  2. For a recurring recipient: open their contact card → Internet format → Send plain text or HTML.

  3. Re-send the message.

Still not working?

If a recipient already has a winmail.dat file they need, free openers exist (search “winmail.dat opener”) — but fix the source; don't make customers install software to read your quotes.

Bosun renders what actually arrived, winmail.dat included, so nobody has to ask the sender to resend it.