UseLegacyFB in Outlook 2007: In-depth look

I would like to continue my series of blogs about various Legacy and Native FB behaviors of Outlook and Exchange. So far we confirmed the following:

1. Outlook 2010 works OK in pure Exchange 2003 environment retrieving FB from Schedule+ F/B System PF. No additional configuration is required like “UseLegacyFB” registry change or anything of similar nature (not to mention that “UseLegacyFB” does not work with Outlook 2010 anyway).

2. Exchange 2010 SP1 goes further in bridging Legacy and Native Free Busy on the server level. While Availability service was always able to go and talk to Schedule + FB PF to retrieve Legacy FB for the mailboxes residing on Exchange 2003, it will not do so for mailboxes on Exchange 2007+ and will not talk to Outlook 2003 (or I should rather say Outlook 2003 will not talk to Availability). So Exchange 2010 SP1 added some additional logic to “RPC Client Access Service” where it answers Outlook 2003 calls into PF and fetches native availability information from the mailboxes.

This new 3rd Free Busy behavior that I want to discuss relates to Outlook 2007 andUseLegacyFB switch. It may confuse some people into thinking that only after setting this switch Outlook 2007 starts noticing Schedule + F/B folder. But the truth is – even without this switch Outlook 2007 will still continue publishing/updating Free Busy into Schedule + F/B System PF like earlier Outlook versions do. The switch actually forces Outlook to readFree/Busy from PFs but regardless of this switch it will still continue publishing Legacy Free Busy if it senses PFs in the environment. This makes it possible for example for Outlook 2003 clients to see Outlook 2007 client’s Free Busy in pure Exchange 2007 org. Below are also some references to Microsoft KB which indirectly talks about this Outlook 2007 behavior.

You can also force Outlook 2007 to behave like previous Outlook versions and use only the account’s Free/Busy message on the Schedule+ Free/Busy public folder. To do this, follow these steps.

3.Locate and then click the following subkey in the registry:
4.On the Edit menu, point to New, and then click DWORD Value.
5.Type UseLegacyFB, and then press ENTER.
6.On the Edit menu, click Modify.
7.Type 1, and then click OK.

While the Free/Busy publishing is performed by Outlook 2007 when delivery to the .pst file is specified, only the Schedule+ Free/Busy public folder message for the user is updated.The Public Folders data does not replicate to the Availability Service. Outlook 2007, as noted above, will query only the Availability Service, when used in conjunction with an Exchange Server 2007 mailbox, as will OWA