There is a long-standing "handshake" issue between Microsoft Outlook and the rest of the internet. Outlook uses a proprietary email format called TNEF (Transport Neutral Encapsulation Format) that most other email programs—including Apple Mail, Gmail, and mobile devices—cannot decipher.
When this happens, the recipient receives a plain text message and an attachment named winmail.dat. Any actual attachments (like PDFs or Word docs) are "trapped" inside that dat file and cannot be opened.
Part 1: How to Fix It (Standard Outlook)
If you are using the classic version of Outlook (2016, 2019, 2021, or Microsoft 365), follow these steps to ensure you are sending emails in a format everyone can read.
1. Update Global Settings
Click File in Outlook and select Options.
Go to the Mail category.
Under Compose messages, ensure "HTML" or "Plain Text" is selected for Compose messages in this format.
Scroll down to Message format.
Under When sending messages in Rich Text format to Internet recipients, ensure Convert to HTML format or Convert to Plain Text format is selected.
Click OK.
2. Clear the Autocomplete Cache (Crucial)
Even if your global settings are correct, Outlook often "remembers" that a specific contact prefers the problematic Rich Text format. If one specific person is still receiving winmail.dat files, you must reset their entry:
Open a New Email.
Start typing the recipient's name or email address.
When the suggestion appears in the dropdown list, click the 'X' to the right of their name to delete it from the cache.
Manually type the full email address the next time you send them a message. This "resets" the preference to your new HTML settings.
Part 2: Modern Updates & "New Outlook"
The "New Outlook" for Windows
If you have toggled on the "New Outlook" switch (the version that looks like the web-based Outlook.com), you will not find the Rich Text settings mentioned above. This version defaults to HTML. If recipients are still getting winmail.dat from this version, the issue is likely being caused by a Microsoft 365 Admin setting (see Part 3) or a copied signature.
Avoid Copy-Pasting Signatures from Word
Creating a signature in Microsoft Word and pasting it into Outlook is a common cause of this issue. Word often embeds hidden "Rich Text" code that forces the entire email into the winmail.dat format.
The Fix: Recreate your signature directly within Outlook's signature editor or use a dedicated HTML signature tool.
Part 3: For IT Administrators (Microsoft 365)
If your entire organization is experiencing this issue, you can fix it globally via the Exchange Admin Center so individual users don't have to change their settings.
Log in to the Microsoft 365 Exchange Admin Center.
Navigate to Mail Flow > Remote Domains.
Edit the Default domain (or create a specific one for the recipient's domain).
Under the Use Rich-Text Format section, select Never.
Save changes. This forces the server to strip TNEF formatting before the email leaves your organization.
Part 4: For Older Versions (Outlook 2007 - 2013)
Select Tools | Options from the menu.
Go to the Mail Format tab.
Under Compose in this message format, ensure HTML or Plain Text is selected.
Click Internet Format.
Under When sending Outlook Rich Text messages to Internet recipients, ensure Convert to HTML format or Convert to Plain Text format is selected.
Click OK and OK again.
Summary Checklist
Is your message format HTML? (Check File > Options > Mail)
Did you copy your signature from Word? (Try sending a test email with no signature)
Does it only happen to one person? (Clear them from your Autocomplete list)
Are you on New Outlook? (Check your Microsoft 365 Admin settings)