Tool: WebMailPRO
Category: Email / Attachments
Updated: 2025
Overview #
Some WebMailPRO users may receive an attachment named winmail.dat instead of the file the sender intended to include.
This file is generated by Microsoft Outlook and Microsoft Exchange when the sender’s email is sent using Rich Text Format (RTF) or when Outlook has been configured to use a Microsoft-only encoding method.
WebMailPRO cannot decode winmail.dat files. The issue must be corrected by the sender.
Why This Happens #
Outlook and Exchange use a proprietary Microsoft format called TNEF (Transport Neutral Encapsulation Format) when sending RTF-formatted messages.
When sent to non-Microsoft mail systems (including WebMailPRO), the message appears as:
- A simplified or plain-text body
- A single attachment named winmail.dat
- The intended files (PDFs, Word documents, images, etc.) embedded inside the TNEF package
Because WebMailPRO does not support TNEF decoding, the original attachments are not accessible.
How WebMailPRO Handles winmail.dat #
WebMailPRO does not unpack, interpret, or extract content from TNEF-encoded messages.
Therefore:
- The winmail.dat file cannot be opened
- The original attachments are not visible
- The message may appear incomplete
The only solution is for the sender to adjust their Outlook or Exchange settings.
Fixing the Problem (Sender Must Make the Change) #
Provide the following guidance to the sender:
Option 1 — Use HTML or Plain Text in Outlook #
- Outlook → File → Options → Mail
- Under Compose messages in this format, select:
- HTML (recommended)
- or Plain Text
This prevents Outlook from using TNEF.
Option 2 — Fix the Individual Contact Entry #
Outlook may store per-contact formatting preferences.
To correct:
- Open the contact in Outlook
- Double-click the email address
- Change Internet format to Let Outlook decide the best sending format
This often resolves repeated winmail.dat problems for specific recipients.
Option 3 — Fix Office 365 / Exchange Online Settings #
The sender’s administrator can globally disable TNEF for external recipients:
- Microsoft 365 Admin Center → Exchange Admin Center
- Mail Flow → Remote Domains → Default
- Set Use Rich Text Format = Never
This ensures all external mail uses HTML or Plain Text automatically.
Temporary Workarounds for WebMailPRO Users #
If the sender cannot change settings immediately, the WebMailPRO user may:
- Ask the sender to resend the file using HTML or Plain Text format
- Ask the sender to upload the file to a shared drive and send a link instead
Security Note:
Various online winmail.dat “extractors” exist, but they may expose private or sensitive information.
We do not recommend using them.
Summary #
- winmail.dat attachments are created by Outlook/Exchange, not WebMailPRO
- The problem must be fixed on the sender’s side
- Once the sender switches to HTML or Plain Text, attachments will appear normally
References / Further Reading (Microsoft & Authoritative Sources) #
- How TNEF affects email messages — Microsoft
https://learn.microsoft.com/en-us/troubleshoot/outlook/message-body/how-tnef-affects-email-messages?utm_source=chatgpt.com - Specify message format for external recipients — Exchange Online
https://learn.microsoft.com/en-us/troubleshoot/exchange/send-emails/how-to-specify-email-message-format?utm_source=chatgpt.com - Why am I getting winmail.dat? — Microsoft Q&A
https://learn.microsoft.com/en-us/answers/questions/4652873/why-i-am-getting-email-attachments-as-winmail-dat?utm_source=chatgpt.com
