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?