Can't connect to external meetings from the Teams Calendar, but can from Outlook

[Brad Groux]

I am getting reports from many of my users across my organization that when they try and join a Teams meeting hosted by an external third party (such as a vendor, partner, or customer), that the meeting never connects if they click Join from the Teams calendar. However, if they go to the same meeting request in Outlook, they are able to join via the web link. 

Is there any particular reason my users cannot join a Teams meeting being hosted by an outside resource directly within Teams? All of our users have been migrated to Office 365 and Exchange Online, and Teams works pretty much flawlessly otherwise. 

RESOLVED - It was Mimecast causing the issue, we simply added a URL bypass policy for teams.microsoft.com to Mimecast, so did not put a mimecast.com security URL in its place.

[#2]

Some further testing has yielded strange results. We've tried connecting to several external tenants, and some work, while others don't. The ones that don't work connect just fine via the Outlook calendar.

[#3 · Accepted solution]

RESOLVED - It was Mimecast causing the issue, we simply added a URL bypass policy for teams.microsoft.com to Mimecast, so did not put a mimecast.com security URL in its place.