Is it possible to direct Free/Busy lookups external for users with EXO mailboxes?

Question

I have an O365 environment where some users are going to be provisioned with Exchange Plan 2 mailboxes. The challenge is that some of them will not use the calendars associated with those mailboxes. Their calendars reside in an external system where we have a successful AvailabilityAddressSpace configured. The external free/busy lookups work fine if a user does not have a mailbox in O365 and is just a mail user object with a TargetAddress that points to the domain referenced in the AvailabilityAddressSpace. This example might make it more clear:

User A has O365 mailbox and uses it for both mail and calendar
User B has an O365 mailbox and only uses it for specific types of mail but never calendar. Their calendar exists in an external system.
User C only has a mailbox/calendar in the external system but has a MailUser (Contact) object in O365 where the target address points to the external system.

I would like user A to be able to pick user B from the GAL when creating a calendar entry and have the free/busy lookup go to the external system.
If User A picks user C it works fine as expected.

Answer 1

Hi,

Based on my knowledge, unfortunately this is not possible since the user B email address is the same domain and the autodiscover/EWS query will be done within the domain and will not check for the availability address space.

May be we look for options like (if any), real time sync between office 365 mailbox calendar and external calendar for user B, so when user A check in scheduling assistant, it will fetch the free busy information from user B's office 365 mailbox calendar which has the updated information from external. Another option, if that external system supports option like sharing calendars, then User B's external system calendar can be shared with User A and sync. However, if the user A still uses scheduling assistant then the free busy will be retrieved from the User B's office 365 mailbox only.

Answer 2

This is may not be possible for user B. Because, when user A tries to access user B's free/busy information, due to user B is the real mailbox in Exchange Online, user A will access user B's Exchange online calendar instead of the calendar that exists on the external system. It is an expected behavior which cannot be changed.

Here are some suggestions which may be useful to you:

• Let User B to use Exchange online calendar.
• Make User B same as User C to be a Mail user in Exchange online.
• Subscribe the external system calendar, then drag meeting from external calendar to Exchange online calendar manually.
• Try to find a tool which could complete the third suggestion automatically.