What you’re seeing
- Outside contacts say they received a file called winmail.dat and nothing else.
- Recipients using Apple Mail, Gmail or older mail programs see plain text with the formatting stripped and the attachments missing.
- Only certain recipients or domains are affected, while colleagues inside the company see the message correctly.
- Meeting invitations sent externally arrive as an unreadable attachment instead of a calendar request.
Common causes
Every environment is different, but these are the causes our engineers find most often:
- Outlook is sending in Rich Text Format, which packages attachments into a Microsoft-only TNEF container that non-Outlook clients cannot unpack.
- A saved contact or autocomplete entry for that recipient is flagged to always receive Rich Text, overriding the normal format.
- The server's remote domain settings allow TNEF to be sent to outside domains instead of converting messages to standard formats.
- A mail relay, signature service or gateway between your server and the internet is mishandling the encoded message.
- A misconfigured add-in or default message format setting in Outlook is causing messages to be sent as Rich Text.
Why it matters
To a customer, a winmail.dat file looks like a broken or even suspicious email. Contracts, invoices and proposals never arrive in usable form, so deals stall and the recipient has to write back asking for a resend. It also makes your business look less professional, and some recipients will simply ignore a message they can't open. Because it only affects certain recipients, the issue can persist for months before anyone connects the complaints.
How AskTechGurus fixes it
Skip the trial and error. A certified engineer works on your actual system, finds the root cause, and fixes it properly.
Tell us what’s happening
Submit a ticket in about two minutes. Paste the error, describe what changed, and pick your tier. No charge to submit.
A certified engineer diagnoses it
A vetted, US-based systems engineer claims your ticket, usually within minutes during business hours, and investigates over a secure remote session you can watch and end at any time.
Fixed and verified with you
The engineer resolves the root cause and confirms with you that it’s working. You don’t just get a workaround, and you get a plain-English summary of what was wrong.
Pay only when it’s resolved
Fixed price from $99, invoiced only after you approve the fix. If it isn’t resolved, you don’t pay.
Frequently asked questions
Is winmail.dat a virus?
No. It is a Microsoft packaging format that holds the original attachments and formatting. It is harmless, but most non-Microsoft mail programs cannot open it.
Is the problem on our side or the recipient's?
Almost always on the sending side. The fix lies in how your Outlook and mail server package outbound messages, so recipients do not need to change anything.