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We have a hosted BitBucket instance that is sending emails via SMTP through Exchange 365 to external users. Some of those users are seeing the properly formatted emails when looking at them in Gmail. Other users see mangled emails that look like they've lost the HTML. Those same users are also seeing winmail.dat attachments.

There seems to be no rhyme or reason to differentiate the users seeing the properly formatted emails and the ones seeing mangled emails. We've checked OS and browser versions. Both sets of users are using the web Gmail UI.

FWIW, the BitBucket users are AD users on an Microsoft Active Directory with external email addresses. Those external users are all at a company that uses Gmail apps. Again, some of those users are seeing the issue and others are not.

Does anyone have any suggestions as to what could be causing this?

  • MIME type can be removed from the email if the email got tagged as spam. – yagmoth555 Oct 12 '18 at 19:30
  • Just to clarify: Company A uses BitBucket and mail is SENDING the same way for everyone: through 365. Company A is also AD integrated, and using Exchange 365 for themselves. Company B is using Gmail, and some users at company B are fine, other users are getting winmail.dat emails. No users at company B are using Exchange for anything. Nobody anywhere is using Outlook for email? – RobbieCrash Oct 12 '18 at 19:30
  • @RobbieCrash - yes, your summary is correct. – Chris Williams Oct 12 '18 at 19:43
  • @yagmoth555 - I'm not seeing anything appearing as spam on the Gmail side. Emails are appearing in users inboxes just like any other email. – Chris Williams Oct 12 '18 at 19:44
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Still not sure what caused this but it seems like 365 decided to send some of our users RTF emails and others it left alone. This seems to have fixed our issue:

Connect to Exchange Online via Powershell and run:

Set-RemoteDomain Default -TNEFEnabled $false

More information on this here:

https://gcits.com/knowledge-base/how-to-fix-the-winmail-dat-attachment-issue/

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