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Configure Organization-Wide Free/Busy Information In An Untrusted Cross-Forest Topology

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Exchange expert Frank Plawetzki recaps a recent rude reminder about the intricacies of Exchange free busy sharing using Add-AvailabilityAddressSpace!

Today I ran into a gotcha, which I thought is worth mentioning.

The customer situation was to connect another company’s Exchange organization, with no trust in place between source and target organization.

I was testing free/busy using OWA, but the user in the remote forest only showed hash marks and their free/busy information was not shown.

The process for configuring free/busy in this scenario is described in Configure the Availability service for cross-forest topologies and in more detail here: Cross-Forest Free/Busy in Untrusted Forests. 

My colleague Neil mentions in the article above, that:

“Note also a key requirement of cross-forest free/busy, namely GAL synchronization. It is absolutely required that GAL synchronization is configured and operational between the two Exchange organizations. Make sure this is the case before proceeding.”

However, as experienced administrators we certainly ignored this recommendation, thinking that this is only a nice-to-have for the users in the source forest to have a convenient way for finding recipients in the remote forest!

We were under the false impression, that you can configure cross-forest free/busy and simply open OWA in the source forest and enter the SMTP address of a user in the target forest and the availability service will use the domain name to query for the target uses free/busy information. 

However, this is not the case. And shows that we have been spoiled with the Microsoft Federation Gateway which does not require GAL Sync. 

If you do not have at least a contact from the target user in your source forest, the availability service in the source forest will not even start querying AutoDiscover in order to find the target address space and you will only see hash marks for the user from the target forest.

So, if you set this up and test it, make sure you at least have a mail contact from the target forest in your source forest when you use a mailbox in the source forest to query free/busy for this contact from the target forest.

Published by MSPFE editor Rhoderick Milne.