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Hello,

We are having an issue where MS Teams Invites are coming in as Plain Text... but not all the time.

When coming in as Plain text, it does not get added to the Outlook/Exchange Calendar. This is an on prem Exchange Server, hosted internally, and not using M365.

It appears random -- different mailboxes/users, different sender. No difference between OWA/Webmail or Outlook Desktop client.

When going to the message ID on the SPAM Server, it contains a winmail.dat attachment. When this is converted, using a free online tool -- the message appears as it should.

Please advise.

Hey there!

Oh, the mysterious winmail.dat strikes again! I've come across this issue a handful of times over the years, and it can indeed be a real head-scratcher.

TNEF & winmail.dat: Microsoft Outlook and Exchange use a proprietary email format called Transport Neutral Encapsulation Format (TNEF). Sometimes, Outlook sends these TNEF-encoded messages out to the world, and other mail clients and systems can't decode them properly. Hence, the appearance of winmail.dat.

Quick Fixes: There are a few things you could try:

Ask the sender to resend the invite/message after ensuring that their Outlook is set to send emails in HTML or Plain Text, rather than Rich Text Format (RTF). Rich Text can sometimes trigger winmail.dat attachments.
On your Exchange server, you can configure Remote Domains to never send in Rich Text, essentially stopping calendar 2024 it from sending these TNEF messages.
Deep Dive: If the issue persists and appears random, you might want to look deeper into the mail headers. Sometimes specific email gateways or security filters can alter the email's format, leading to these inconsistencies. If EFA (Email Filter Appliance) is altering or misinterpreting these headers, then adjusting configurations or setting up exceptions might be the way to go.

Lastly, you may want to consider getting in touch with EFA support or diving deep into their documentation if you believe the issue might be on that end. The problem with winmail.dat is that it often requires a bit of detective work to find out where the problem truly lies.

Best of luck, and I hope you find a resolution soon! And remember, you're not alone in the winmail.dat battle - many of us have been there before.