PROBLEM
For some emails, the recipient receives a text message with a winmail.dat attachment, no HTML part.
SOLUTION
These emails are in an old Microsoft proprietary format called TNEF: the email content is placed inside an attachment named winmail.dat.
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
On M365 go to https://admin.exchange.microsoft.com/#/remotedomains and set "Use rich-text format" as "Never".
Once disabled, Microsoft will always use the standard email format instead of this TNEF format.
Exchange On-Premise
In order to disable it, the following command must issued on the console:
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.
IF SOLUTION DOESN'T WORK
Please contact the support at libraesva.freshdesk.com and provide screenshot of the configuration performed. We will promptly assist you to solve the issue and receive these kind of emails.