PROBLEM
For some emails, the recipient receives a text message with a winmail.dat attachment, no HTML part.
SOLUTION
The first step would be to disable this format on the mailserver, because it creates operability issues with many non-Microsoft systems, with gateways and with recipients who do not use Microsoft software.
- Microsoft 365 Online
- Exchange On-Premise
Once disabled, Microsoft will always use the standard email format instead of this TNEF format.
Set-RemoteDomain Default -TNEFEnabled $false
Please note that disabling this option on your own mailserver affects also email delivered to you from third parties. Microsoft uses some kind of heuristics based on the previous communication history to decide whether an email for a specific recipient should be sent as a TNEF email or as a normal email. If you disable TNEF for your users, after a while you will see a decrease in inbound email using TNEF.