Plain text emails sent from Office 365 have Content-Type: application/ms-tnef; name="winmail.dat"

Hi, I have found that with every email sent from a mailbox on my Office 365 for Business account, if I set the email to be plain text, in the header I get - Content-Type: application/ms-tnef; name="winmail.dat" - this happen regardless of whether I send from Outlook 2013, or OWA. The problem is that because of this the message is getting flagged as not being plain text with a 3rd party system. Emails sent to the internal inboxes that are set as plain text come through as plain text just normal, but if the message originates from within the domain, then I get the winmail.dat.

Content-Type: application/ms-tnef; name="winmail.dat"
Content-Transfer-Encoding: binary

Plain text emails from external sources (on premise exchange and gmail) the header looks correct:

Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: quoted-printable

Which leads me to think that it's the Exchange Server at Office 365 that did the conversion. I don't want it to do any conversion though--if the email was sent as plain text, I want it to stay that way.

Any suggestions for how to correct this?


Question Info

Last updated June 1, 2020 Views 1,280 Applies to:

Hi Marc-565,


Thanks for your information. Based on my experience, it’s not feasible to change the settings in mail header. In order to help you solve this problem, I recommend that you seek help from your 3rd party system and check if any settings on their side can help you. 


Thanks for your understanding.



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