Here is how to prevent Outlook to sent winmail.dat attachments on iPhone

When sending e-mails from Outlook to iPhone, some of the receivers (who use iPhone) may complain to you that they receive a winmail.dat file they can’t open on their device. Or perhaps you’ve sent e-mails to yourself from an Outlook mail client, and wanted to open that e-mail on your iOS device and you saw the winmail.dat attachment at the bottom.

Example of a winmail.dat file attachment on iPhone

Why do I see winmail.dat as attachment?

The Outlook program is created by Microsoft. Microsoft formats e-mail sometimes as RTF (rich text format), which allows you to use for example bold text. Unfortunately, your iPhone doesn’t know how to handle this and will thus create the winmail.dat file attachment.

Prevent winmail.dat attachments

To prevent winmail.dat attachments, you should use HTML or plain text instead of RTF e-mail messages. Here is a step by step explanation on how to make sure that every e-mail you sent in Outlook is sent as an HTML e-mail.

  • Open Outlook on your computer
  • Go to Options and click on Mail
  • In the Compose Message section, there is the option ‘ Compose messages in this format’: make sure it is put in HTML or plain text
  • In the Message Format section, the option ‘ When sending messages in Rich Text format to Internet recipients‘ should contain the value Convert to HTML format.

Open winmail.dat on iPhone via an app

Preventing winmail.dat attachments is something you can control when you are the one that sends e-mails. However, sometimes you are at the receiving side of things. In this case, you may still want to be able to open that winmail.dat file attachment without necessarily bothering the sender.

There are plenty of free and paid apps on the App Store. Often, the free versions will contain some sort of advertisement. Search for ‘winmail.dat’ on the App Store and you will be able to select from multiple apps. Choose an app that has good ratings and reviews on the app store and try it out.


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21 thoughts on “Here is how to prevent Outlook to sent winmail.dat attachments on iPhone

  1. Since a month or so, I also have this problem.
    Instructions above have been checked but the problem is still there. Any other suggestions? Annoying …

  2. I’m having the same problem and it’s only recent. I haven’t changed any settings so I don’t know why this is happening. If I send an email with an attachment from my laptop using Outlook to my iPhone, it opens fine in my icloud email but on my other email it comes through as a winmail.dat attachment. This is not happening on my laptop, the attachments are fine. I have checked in the COMPOSE MESSAGE section that messages in Outlook are being composed in HTML and in MESSAGE FORMAT they’re being converted to HTML. They were already on these settings so I don’t know why I’m suddenly having this problem.

    1. Same problem didn’t change any settings if I send to a friend he can open it. What the heck???? It’s not my computer. I think it’s Apple???? Or a bug.

  3. My wife has the exact same issues when sending an attached JPEG image…all settings set to HTML as suggested. The interesting thing is that when she sends it to my iPhone or iPad I get the JPEG, but when sent to her own iPhone or iPad she get the .dat.

    These are the exact same model/ Mail version of both our iPhones and iPads. same build of Outlook and same IP. I suspect it has something to do with the recent Apple updates to both our phones and iPads.

  4. why doesn’t any reply to help. I have the same issue and my laptop is set to HTML in both places. This only started in the past 3 weeks. It’s very frustrating!

  5. I had this same problem in Outlook 2019 and what ended up fixing is ensuring what this article says
    1. Ensure the Options>Mail>Compose Messages is HTML
    2. Ensure the Options>Mail>Message Format is “Convert to HTML format”

    But then also I needed to go into a new email and put in the mail recipient in the TO box and then RIGHT click email message for the person and change the Internet Format” in the email properties to “Let outlook decide the best sending format”.

    Now try to send something to that person.
    This fixed it for me… and the setting seemed to be picked up by other email I sent…so all resolved.

    1. Compose a new email, but before you send it, right click on the email address of the recipient. Select open Outlook properties. In the box for Internet format, select the drop-down arrow and select “let outlook decide the best sending format.” Click OK. I sent an Excel file and the recipient was able to view it in excel on their iPad.

  6. Thanks for all the info.
    My office 365 was set up with HTML and the attached .pdf came across as winmail.dat.

    I switched to Plain Text Format and it was received as a .pdf

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  8. Unfortunately, Esa and Lisa’s solution does not work in Outlook 2021 (standalone version). Selecting recipient email address does not give you an option to open Outlook Properties.
    Any other solutions?

  9. Hi Seth, Like you, I have Outlook 2021 which I purchased and have been sorry for it ever since. This problem has been doing my head in. I even tried deleting the new version and reverting to 2010 version but was prevented from re-installing it. Since installing the 2021 version, all my friends have been experiencing difficulties opening attachments (in IOS, Android, and Windows 11 environments) I think, it is when they do not have Outlook on their PC/device.
    In any event, following Esa and Lisa’s solution, I was able to select the recipient email address by ‘right’ clicking on it. From there, the Address details and a dropdown box containing the option “Open Outlook Properties” displayed. Selecting the latter enabled me to then choose the Internet Format: “Send Plain Text Only”. And with that, the problem was corrected BUT only in that instance. The change has not affected subsequent transmissions ie: I have to change the Internet Format each time and for each recipient which of course is a real pain if sending to multiple recipients.

  10. It seems this is also related to automatic filling of email addresses in the send to field. I’ve had success by taking steps above…

    1. Ensure the Options>Mail>Compose Messages is HTML
    2. Ensure the Options>Mail>Message Format is “Convert to HTML format”

    Then in File/Options “Send messages” click on the “Empty Auto-Complete List” and to start fresh to add auto complete back, check the box to “Use Auto-Complete List to suggest names when typing in the To, CC and Bcc lines”

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