Securing External Meeting Requests for Microsoft Teams Rooms

Last Updated on March 23, 2021 by GrahamWalsh

I get the question many times about Securing External Meeting Requests for Microsoft Teams Rooms. This is one of the requirements for Guest Join Access for Microsoft Teams Rooms. I knew there must be a way to do this, so after some research I discovered you could use Transport Rules in Exchange, now known as Mail Flow Rules. Full reference here.

So, I thought I’d record a video overview on how to set this up. See below.

In summary, the user how wants their room resource to be secured, you just need to add some rules. You open the mailbox to allow external meetings (details here) but then secure it via the Mail Flow rules.

In the example below, I simply say which recipient this rule applies to (optional) and then say the message must originate from outside the organisation. Then I can say that the message header must include calendaring (even more locked down). I also have a bounce back message if someone fails the check (see further down). Finally, I say which domain(s) can bypass this rule. In my case, it’s my own domain can send external invites to my test tenant.

Exchange Mail Flow Rules

If someone fails the Mail Rules, they then receive this error message when they try and invite from external.

Blocked by Mail Flow Rule

So, there we have it, a way to limit the exposure of external senders to your meeting room devices/calendars by Securing External Meeting Requests for Microsoft Teams Rooms.

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