I was receiving an error indicating "Database is mandatory on UserMailbox" when trying to install exchange 2013 in an exchange 2010 sp3 environment.
I had to remove the federated mailbox in ADSI and re-add it.
The instructions for repairing this are provided by Microsoft but they are out of date and order: Old instructions --> http://support.microsoft.com/kb/978776
I've correct 2013 instructions and posted below for anyone who needs to resolve this.
Remove the FederatedEmail.4c1f4d8b-8179-4148-93bf-00a95fa1e042 e-mail account by using the ADSIEDIT tool. To do this, follow these steps:
1. Click Start, click Run, type adsiedit.msc, and then click OK.
2. Locate the Default Naming Context node, and then locate to the CN=Users container.
3. Locate and then right-click the CN=FederatedEmail.4c1f4d8b-8179-4148-93bf-00a95fa1e042 container. Then, clickDelete.
4. Create a new federated e-mail account by using the following command in the (newly installed) exchange management shell:New-Mailbox -Arbitration -Name FederatedEmail.4c1f4d8b-8179-4148-93bf-00a95fa1e042 -UserPrincipalName FederatedEmail.4c1f4d8b-8179-4148-93bf-00a95fa1e042@<Default_Accepted_Domain
5. Rerun the Exchange Server 2013 Mailbox role setup application.