Calendar Sharing Between Exchange Online Tenants

The rapid adoption of Microsoft Office 365 services globally has changed the landscape of how organizations collaborate across traditional corporate boundaries. Many organizations who undergo a large amount of acquisitions now have new considerations when it comes to migrating cloud services. While migration is the only fully featured solution for bringing two disparate tenancies together without any “rough edges”, there are still a lot of ways to collaborate with other tenancies before a migration takes place, or where a single tenancy is not appropriate.

While there are a lot of relatively new external collaboration features and concepts such as Azure AD B2B, in this post I’m going to look at something that has been around for a long time, cross organizational calendar sharing. Here, I’ll go through the (relatively straightforward) steps involved to set up calendar sharing between tenancies and some of the considerations to keep in mind when setting it up.

Before you start

Before starting, some concepts to understand:

  • Organization Relationship – An Organization Relationship is a one-to-one relationship between Exchange Organizations allowing users from each organization to view and share calendar information (such as free/busy information). This works on an organizational level
  • Sharing Policy – A sharing policy enables the ability to share calendars with external users. By default it is possible to share calendars with any domain (or anonymously). I recommend removing these default settings and opening up to specific domains when required.

Create an Organization Relationship

To create an Organization Relationship, navigate to the Exchange Admin Center and open the “Organization” -> “Sharing” page. From here, under the “Organization Sharing” section, click the ‘+’ button to create a new Organization Relationship. Add in a name for the relationship and the domain(s) that you want to establish the relationship with. Next, choose the level of detail that will be available over the relationship and optionally specify a group to limit the scope. When setting up an organization relationship, you only need to add a single domain from the tenancy and Exchange Online will find the rest. You can also find the other domains yourself by using the Exchange Online PowerShell cmdlet “Get-FederationInformation -DomainName <DomainName.com>” (Figure 1).

Figure 1: The result of Get-FederationInformation shows the domains available for Federation

When the settings of the relationship are in place (Figure 2), hit ‘Save’ to start the creation process. This process should also be carried out on the other tenancy to allow two way federation.

Figure 2: Configure the Organization Relationship

When the relationship is saved, you’ll see that any other domains identified in the “Get-FederationInformation” cmdlet will also be added to the relationship (Figure 3).

Figure 3: Exchange Online will detect the federated domain list for you when you specify one domain

When the relationship is configured on both sides, users within the scope will be able to perform lookups and see Free/Busy information (Figure 4). Note, this doesn’t allow sharing of calendars or visibility in the GAL.

Figure 4: A cross-tenant Free / Busy lookup now shows information from the external users calendar

Configure a Sharing Policy

As shown above, the Organization Relationship is only one part of the puzzle. If you want to allow users to share out their calendars with external users, you need to set up a Sharing Policy. To do this, open the same “Sharing” page as before and create a new Sharing Policy (Or edit the default) under the “Individual Sharing” section (Figure 5). If you create a new policy, remember to assign it to users before testing.

Figure 5: Create or edit a Sharing Policy

I highly recommend modifying the default policy to remove the sharing capability from the “Anonymous” and “Sharing with all domains” sharing rules (Figure 6) to prevent uncontrolled sharing.

Figure 6: Remove the sharing capabilities from the “Anonymous” and “Sharing with all domains” rules

Within the policy, create a new sharing rule, specifying the domains to share with and the level of sharing to support as shown in Figure 7, and then save the policy. If the policy isn’t the default, make sure you have assigned it correctly to users.

Figure 7: Configure a sharing rule for the partner domain(s)

With the policy in place, users can now share their calendar with users in the external domain, just like they would internally (Figure 8).

Figure 8: Share a calendar externally

The external user will now get an invite to open the shared calendar as shown in Figure 9.

Figure 9: The external user will receive a calendar invite

Accepting this invite will allow the user to view calendar information across tenants (Figure 10).

Figure 10: The external user can see the calendar across tenants

It’s important to note that instant update of calendar entries is not currently supported between tenants but there are constant improvements being made to the sharing model so hopefully soon this will be in place!

Summary

Calendar sharing isn’t anything new or particularly fancy but I’m surprised at how many organizations I talk to that either aren’t aware it can be done in Exchange Online, or aren’t aware how easy it is to set up and manage. With all the exciting B2B collaboration features available in Microsoft 365, it’s easy to forget about the basics.

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