Ever wondered about people complaining that they got weird attachment with your email named WinMail.dat, they even try to open it but it didn’t open, actually they didn’t have a program capable to open that file. It was the Job of their email client (Mac Mail, Mozilla Opera) whatever they were using and that’s the sole reason that it removed all the styling which you did in your brand new Microsoft outlook and attached it as a strange file called WinMail.dat and presented the user with a dull  text only email.

Here is what Microsoft says about it:

By default, email messages that are sent from Exchange Online in Office 365/Outlook 2013/Microsoft Exchange Server use the Transport Neutral Encapsulation Format (TNEF) format. Messaging systems that aren’t based on Microsoft Exchange may be unable to interpret messages that use this rich text format. If the recipient’s messaging system can’t process this format, a file attachment that’s called Winmail.dat is added to the message.

So now that we know about the issue lets talk about the solution. Well as we have already explained that the issue is not with your computer but with the outdated email client that Recipient using. What needs to be done is that you send him an email in plain text which his email client can understand.  So we have a couple of options here. If the recipient is someone to whom everyone (in your company) sends email and its an issue with everyone then we can do a change on the exchange server itself or if its only your problem then you can make changes to your personal outlook to fix the issue or other wise we can make the change on the Microsoft Exchange Server Itself to fix the issue.

There are numerous articles written about how to fix it on outlook at outlook Level, Google it and you will find one.

However if we want to make a system wide solution on your personal Exchange Server or Office 365 servers, we again have two options

1) Resolve the issue with an individual External User or Contact, so here is what we need to do.  Open Exchange Management Shell or Incase of Office 365 connect to MsExchangeonline Shell (Refer here ), and execute the following commands

Set-MailContact  -UseMapiRichTextFormat Never

Run the following Windows PowerShell command to confirm that the message format was applied:

Get-MailContact | Select  | Select UseMapiRichTextFormat

2) But if it’s not about a certain contact rather its a whole domain (May be some other company, your company does business with) then let’s do the following to ensure that our Exchange Server sends non TNEF email to every user of that domain.

Run the following Windows PowerShell command to create a remote domain for an external domain:

New-RemoteDomain -Name  -DomainName

Run the following Windows PowerShell command to prevent messages from being sent in rich text format:

Set-RemoteDomain -Identity  -TNEFEnabled $false

Run the following WindowsPowerShell command to check that the setting was applied:

Get-RemoteDomain -Identity | Select TNEFEnabled

Hope this article helps you, else feel free to reach us for paid support options here


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An automobile enthusiast at heart and computer geek by profession, started my Career with MS in 2005.Left Jobs and started Pledge Technologies (the parent company to Grishbi) back in 2009.We have been providing IT consulting to various Small and Medium businesses across US and UK since then.Our company specialises in Microsoft Server technologies like AD, Exchange, the rest and with numerous Office 365 migrations under our belt, we quite an expert with that too. Whatever we learn in our day to day life, we share it back on Grishbi as a Thank for all the love and support our customers have given us.
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