🔧 Technical Context
In some configurations, the Outlook email client may send messages using the proprietary RTF (Rich Text Format).
This format is only interpreted by Microsoft products (such as Outlook or Exchange) and is therefore not compatible with most other email clients.
When a message is sent in RTF format, its content (text formatting, fonts, colors, attachments, etc.) is encapsulated in a TNEF (Transport Neutral Encapsulation Format) file, usually named winmail.dat.
This file contains all formatting elements and attachments, but it cannot be read by non-Microsoft applications (such as Thunderbird, Pronto, Gmail, etc.), which then display it as an unreadable attachment.
👉 To ensure full compatibility with all recipients, it is recommended to configure Outlook to send messages in HTML format.
This standard format is recognized by all email clients and preserves formatting without generating a winmail.dat file.
✅ Resolution
1. Open Outlook settings
Open Outlook.
Go to:
File → Options → Mail
2. Message composition settings
In the “Compose messages” section:
Field:
Compose messages in this format
Possible values:
HTML → standard format, universally compatible
Rich Text → proprietary Outlook format
Plain Text → no formatting, 100% compatible
➡️ To prevent the creation of a winmail.dat file, select HTML (recommended) or Plain Text.
3. Conversion settings
Scroll further down on the same page to the “Message format” section:
Field:
When sending messages in Rich Text format to Internet recipients
Possible values:
Send using Outlook Rich Text Format → generates a winmail.dat file
Convert to HTML format → no winmail.dat
Convert to Plain Text format → no winmail.dat
➡️ To force sending without a winmail.dat file, select Convert to HTML format (recommended) or Convert to Plain Text format.
📊 Cross-test results table
⚙️ If the problem persists...
1. CLEAR THE RECIPIENT CACHE (“To” field) to avoid winmail.dat
Sometimes Outlook stores an RTF configuration associated with a specific email address.
To clear this cache:
In a new message, click in the To field
Start typing the affected recipient’s email address
When the name appears in the suggestion list, hover over it
Click the X on the right side of the name to remove it from the cache
Close and reopen Outlook, then manually re-enter the recipient’s full email address
➡️ This forces Outlook to recreate the contact without the old RTF configuration.
2. DISABLE RTF FORMAT IN CONTACT SETTINGS
Open the contact → right-click → Outlook Properties → select Send Plain Text only or Let Outlook decide the best sending format (HTML preferred).
➡️ This changes the sending preference for this contact and prevents Outlook from using RTF format by default.