I've been having an issue where external Teams-enabled meeting invites are no longer displaying correctly. Instead, I receive a notification stating "not supported calendar message.ics." This started happening yesterday, and we are using Exchange Online. I've checked and confirmed that standard invites from M365, Gmail, and internal meetings come through just fine.
I even downloaded a test email with a Teams invite from outside my organization and inspected the .eml file, which looked as it should. However, when the email arrives in any of the supported clients—Classic Outlook, Web Outlook, or Outlook Mobile—it gives the error message instead of the usual meeting invite display.
We have Mimecast as our email gateway, but I haven't made any policy changes recently. It seems odd that the .eml file retrieved from Defender shows the correct details. Has anyone else experienced this issue?
3 Answers
Thanks for sharing that! I’ll definitely look into submitting my own ticket. The export from Defender was throwing me off!
It looks like this might be an issue related to Mimecast. I've read people are encountering the same problem, and it seems the integration with Teams invites is having glitches. Have you looked into the settings in Mimecast or checked for any recent updates that might affect this?
Not necessarily a Mimecast issue on its own, since I don't use it and I'm experiencing the same problem. It definitely seems connected to the Teams setting.
This worked for us: check out this guide on the Mimecast support page regarding Microsoft Teams. It provided some clarity on updates we needed to implement since the issues started cropping up.
I don't think it's just a Mimecast problem. I tested sending an invite from my Hotmail, which isn't affected by Mimecast's URL rewriting, and still got the same "not supported calendar message.ics" error. So it might be something more specific with the Teams option being enabled.