The Lotus/IBM Notes can receive large attachments in their emails and they belong to different formats for images, documents, sheets, and videos. But, a dreaded attachment the belongs to RTF format shows it as ‘Winmail.dat’ and it does not show the correct format also. It is actually an encoded message has the plain text of the message and the binary attachment with winmail.dat format. It has the following information-

  • Only text of the message.
  • Embedded pictures and MS Office documents.
  • Custom features and special links and buttons.
  • Regular attachments.

The winmail.dat does not open in the existing application that open the supposed attachments and there can be numerous reasons behind such problem.

Why this Winmail.dat Attachments Issue?

This attachment issue basically arises when the Exchange Server sends a message without turning ON the MIME encoding and has adopted the proprietary of Rich Text Format (RTF). Lotus Notes application does not support messages in Rich Text Format in its older versions, although the latest one like Domino 8.5x supports the RTF format.

You can either try finding the solution through Domino Administrator or through changing RTF formatting in different versions of Exchange Server.

How to Resolve Winmail.dat Attachments Issue?

First, install the latest updates for Domino (versions 6.5.6, 7.0.3, 8.0.2, or 8.5.x).

Open your Domino Administrator. Navigate to SERVER > SERVER CONSOLE. Then run these commands one after another.

set config TNEFKeepAttachment=1

set config TNEFEnableConversion=1

tell router update config

Modify Exchange Rich-text Format Settings:

  • For Exchange 2003 :- Launch the Exchange Server Manager. Go to Global Settings in the Exchange System Manager, and then select Internet Message Formats and then click Standard.
    In the context menu, select Properties and click on the Advanced tab. Now under the heading "Exchange rich-text format" select 'Never Use'option and click on OK. Open the property of the domain and go to Advanced
  • For Exchange 2007 :- Open the Exchange Management Console. Then under Open Organization Configuration, select Hub Transport. Click on Remote Domains, then right click on Default Domain option and select Properties. Click on Format of original message sent as attachment to journal report option and click on the Message Format tab. Now within the Exchange rich-text format section at the bottom, select Never use and click on Apply.
  • For Exchange 2013/2016/2019 :- In Exchange Management Shell, run the following command to list the domains with RTF (TNEF) encoding.
Get-RemoteDomain | Where {$_.TNEFEnabled -ne $false}

Disable sending messages in RTF domain by running this command.

Set-RemoteDomain -Identity DomainName -TNEFEnabled $false

After executing the commands, you need to restart the Microsoft Exchange Transport Service to confirm the changes.

Configure TNEF conversion options

Mail senders can control the conversion options for TNEF messages sent to all recipients (both internal and external). The options available are:

  • Convert to Plain Text format
  • Convert to HTML format
  • Convert to Rich Text format Message format settings

All these RTF formatting settings would resolve the WinMail.dat attachment issues in non-Exchange based email clients like Lotus Notes. And if you find many restrictions or limitations in your current Lotus Notes email client, you can anytime migrate its data to any version of Exchange Server with Kernel for Lotus Notes to Outlook tool. The software has the simplest user-interface and performs automated migration in no time. Feel free to migrate anytime with secure software.

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