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Is your Outlook creating winmail.dat attachments with every email you send? Believe it or not, this is a very common problem and here’s how to fix it!
To stop Outlook from sending winmail.dat attachments, set your new emails to compose in HTML or Plain Text format. Also, when sending Rich Text emails to internet recipients, make sure they are converted to HTML format or Plain Text format.
If you aren’t sure how to do that, follow our instructions below.
When you use Microsoft Outlook together with Microsoft Office 365, you experience one or more of the following symptoms: Outlook responds slowly when you try to open email messages. When you send an email message, the message sits in the Outbox folder for a long time. Outlook responds slowly when you try to insert an attachment. Migration Environment: SP 2007 to Office365 Issue: There is a List is having attachments(.htm &.html) files in SP2007, while moving the data to Office365.The attachments(.htm &.html) will be downloaded instead of opening in the browser which is default O365 Behavior. In the case of Office 365, note that external sharing may be switched off, in which case links will not work. External sharing may also be disabled for specific sites. Maybe Outlook 2016 is smart enough to detect whether or not external sharing is enabled, but if so, this does seem to go wrong sometimes.
Outlook will use your default browser to open or preview many types of attachments. To change your default browser, see Change your default browser in Windows 10. When using Save All Attachments to a folder if a file with the same name exists Outlook creates a new file Filename (1), Filename (2), etc.
Why does Outlook send winmail.dat attachments?
When composing an email in Outlook, there are 3 ways to format your email: Plain Text, Rich Text, or HTML. By default, Outlook uses HTML format when composing new emails.
The winmail.dat attachment is added to Outlook when you send an email using the Rich Text format that has text enhancements such as bolding, italics, font sizes or any other features that are available with Rich Text format. The file is attached to the email because it contains instructions on how other email clients should format the text.
This only happens when composing in Rich Text format because Plain Text format doesn’t have the ability to add text enhancements, and HTML format includes all the enhancements in the HTML code itself, so there is no need for a winmail.dat file.
The winmail.dat problem happens with all versions of Outlook from 2003 onwards.
How to Prevent Winmail.Dat Attachments from Being Sent in Outlook
Step 2
Click on Mail from the left menu.
Under the Compose messages section, change the “Compose messages in this format” dropdown box to HTML or Plain Text.
Step 3
Scroll down until you see the Message format section.
In the “When sending messages in Rich Text format to Internet recipients”, ensure you select either Convert to HTML format, or Convert to Plain Text format.
Click OK to close the Options window.
Preventing Winmail.Dat Attachments in Outlook 2003 – 2007
Step 3
Under Message format, change the “Compose in this message format” dropdown box to HTML or Plain Text.
Click OK to close the Options window.
Using the Windows Registry to Stop Winmail.Dat Attachments from Being Sent
Warning: Editing your Windows registry incorrectly or making a mistake can cause irreversible damage to your operating system. It’s not advisable to edit your registry manually unless you absolutely have to. Only use this method if the above instructions don’t work and you’re comfortable editing the registry.
Step 1
Ensure all applications on your computer are closed, including Outlook.
Right click on the Windows logo in the bottom left corner, click Run.
Alternatively, press the Windows key + R to open the Run box.
Step 2
In the text box, type “regedit” and press Enter, or click OK.
Step 3
Navigate to the following path:
HKEY_CURRENT_USERSoftwareMicrosoftOffice<office version number>OutlookPreferences
The <office version number> corresponds to the following versions of Microsoft Office:
Outlook 2019/2016 – 16.0
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Outlook 2013 – 15.0
Outlook 2010 – 14.0
Outlook 2007 – 12.0
Step 4
Right click on Preferences, then select New > DWORD (32-bit) Value.
Step 5
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The name for the new registry key should be “DisableTNEF”. Once you have added the name, hit Enter to apply the changes.
Step 6
Microsoft Office 365 Outlook Help
Right click on the new DisableTNEF registry key and click Modify.
Step 7
Set the Base to Hexadecimal.
Set the Value data to 1.
Click OK.
Close the Registry Editor window, and you’re done! Outlook should no longer be sending winmail.dat attachments. Django serialize dict object has no attribute meta.
If you’re still experiencing winmail.dat problems…
If you’ve completed the above steps and you’re still experiencing winmail.dat problems, you’ll need to remove the autocomplete entry for all of your recipients. Autocomplete is when you start typing an email address in a new message, and their email address comes up as a suggestion.
The sending format is saved for every contact in your autocomplete list. So, even after you’ve changed the sending preferences using the above instructions, some recipients that you’ve previously sent emails to may still get the winmail.dat attachment if you dont clear the autocomplete list.
Anyone who works with Office 365 knows that there is no shortage of new features rolling out, the pace at which new functionality is made available definitely keeps you on your toes.
Part of what inspired me to develop www.roadmapwatch.com is that I wanted to know more about when features progressed through the various stages on the official Office 365 Roadmap. Even with that tight watch of the roadmap, there are 164 features currently in some sort of “in progress” state and it’s hard to track them all.
On top of the features documented on the roadmap, there are occasionally small items that either slip through the cracks or aren’t worthy of a roadmap mention. One of those features is the “Common Attachment Blocking” feature in EOP that was introduced some time in the last month or so.
Below is a summary of what “Common Attachment Blocking” is all about…
Part of what inspired me to develop www.roadmapwatch.com is that I wanted to know more about when features progressed through the various stages on the official Office 365 Roadmap. Even with that tight watch of the roadmap, there are 164 features currently in some sort of “in progress” state and it’s hard to track them all.
On top of the features documented on the roadmap, there are occasionally small items that either slip through the cracks or aren’t worthy of a roadmap mention. One of those features is the “Common Attachment Blocking” feature in EOP that was introduced some time in the last month or so.
Below is a summary of what “Common Attachment Blocking” is all about…
The Timeline
Office 365 Outlook Attachment Problems Today
Chatter about “Common Attachment Blocking (CAB)” started on one of the EOP blogs back around August 2015. In January of this year, there was a mention on a different EOP blog and on the Office blog that the feature would be coming in “the next quarter”. And then… Well, that was it. I never saw another mention of the feature or it’s rollout status.
It turns out that the feature was released in the last couple months and you’ll likely find it available in your tenant right now.
It turns out that the feature was released in the last couple months and you’ll likely find it available in your tenant right now.
A “New” Feature?
Office 365 Outlook Attachment Problems 2016
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There’s always been a way to block attachments by extension in EOP via a transport rule. However, using a transport rule gave you somewhat limited options when it came to the user experience. You could reject or delete a message with an attachment but there wasn’t a clean way to just strip the attachment and send the message along to the end user.
Using “Common Attachment Blocking”
You’ll find CAB buried in the Anti-Malware Filter Policy in EOP. From the Exchange Admin Center, if you navigate to “Protection” and then “Malware Filter”, you’ll see your default policy. On the “Settings” tab is the option to enable CAB; despite being “recommended”, it will be disabled by default in your policy.
Once enabled, there is a default list of 10 file extensions that Microsoft has selected and you can add more from a pre-defined list of 96 file extensions. All your favorites such as .exe, .com and .vbs are there.
Once enabled, there is a default list of 10 file extensions that Microsoft has selected and you can add more from a pre-defined list of 96 file extensions. All your favorites such as .exe, .com and .vbs are there.
TIP:While you cannot add custom extensions via the portal, it does appear that you can use the “-FileTypes” switch on the “Set-MalwareFilterPolicy” cmdlet to add extensions not in the list of 96.
User Experience
Any attachment file extension that you’ve selected will trigger the “Malware Detection Response” in your policy. You have the option to delete the message in it’s entirety or you can replace the attachment with a text file containing a notification.
The default notification looks like this:
Otherwise you can provide custom text in the notification.
The default notification looks like this:
Otherwise you can provide custom text in the notification.
Testing It Out
You can easily test out the feature by creating a second anti-malware policy (you’ll find CAB enabled by default on it) and applying it to only a subset of users via the options on the “Applied To” tab of the policy.
Give it a try! Let me know what you think in the comments below.
Did you find this article helpful?
Leave a comment below or follow me on Twitter (@JoePalarchio) for additional posts and information on Office 365.
Looking to do some more reading on Office 365?
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Give it a try! Let me know what you think in the comments below.
Did you find this article helpful?
Leave a comment below or follow me on Twitter (@JoePalarchio) for additional posts and information on Office 365.
Looking to do some more reading on Office 365?
Catch up on my past articles here: Joe Palarchio.