Exchange Federation Services and Free/Busy

Exchange Federation Services and Free/Busy limitations for linked mailboxes

Exchange federation is supported and well documented however Free/Busy information for linked mailboxes seems like a grey area. I was recently approached to advise customer to provide remediation advise on lack of Free/Busy for linked mailboxes between two Exchange 2010 organisation.

I was told that the availability service was working for user mailboxes in both organisations except for linked mailboxes. I started gathering as much information as possible about the set-up. Both organisations were running Exchange 2010 SP3 and availability across both organisations was in place through federation trust – Microsoft Federation Gateway. Figure 1 shows cross organisational availability through simple federation gateway set-up.

Fig 1 Cross Organisational Availability Service

There are plenty of online resources from TechNet to blogs when it comes to configuring organisational availability service using Microsoft federation gateway and organisational relationship but unfortunately none of this information really covers  free/busy availability for linked mailboxes.
As we know federation is limited to Free/Busy and is not always a perfect solution especially if you are running anything less than Exchange 2010 or full Calendar availability is required. You have to rely to 3rd party solutions to overcome  some of these limitations.
This blog does not deal with instructions on configuring federation services or 3rd party solution or suggesting which solution is right for you – but I am only going to shed some light on lack of Free/Busy for linked mailboxes when using federation services.
In the beginning I wasn’t quite sure for the purpose of the linked mailboxes and neither I had any details on federation set-up. Other than that I wasn’t sure if Free/Busy was actually supported for linked mailboxes and to what extend? I turned to my senior colleague and then to our Microsoft partner portal for clarification. The answer I have come up is based on R&D and applies to availability services between two exchange 2010 organisations. Now take bit of time to look at this visual diagram for Free/Busy supportability between linked mailboxes.
There is a big change in set-up if you haven’t notice already and this is “introduction of cross forest trust”. To have Free/Busy for linked mailboxes available – you are also required “Cross Forest AD Trust” where “DomainA” trusts “DomainB” and vice versa. This is a quite big change and may not go so well for those organisations who wanted to share F/B and not having to worry about trust issues.
What do you think about the linked mailboxes – perhaps not so great for sharing Free/Busy information despite having “Cross Forest AD Trust” in place. To me it is obvious that federation alone will not provide free/busy availability for your linked mailboxes and even after you have taken steps to implement forest trust – there is no guarantee that Exchange user base between two organisations will have great experience due to these limitations.
The above self-explanatory visual diagram shows that Free/Busy is available for linked mailboxes where a “Mailbox” is in “ForestA” and corresponding users in “ForestB” and vice versa. However, a “Linked Mailbox” in “ForestA” is unable to talk to a “Linked Mailbox” in “ForestB” for Free/Busy. I am not sure if there is a way get cross talking between linked mailboxes (in red Bi-Directional Arrows) working. I will be interested to hear your real world experiences and thoughts on this.
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A father of a beautiful daughter Ayat and husband to my wonderful wife Anjum, living in a beautiful coastal line of Dorset (England) and work as a Microsoft Consultant Engineer. I have over 15 years of IT experience in maintaining and troubleshooting infrastructure environments. I am an SME for Microsoft Exchange, Active Directory, Office 365 and Hyper-V.

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