Consider the following scenario:
In a Microsoft Exchange Server 2010 environment, you create a shared mailbox whose SMTP address starts with "oma" (as in the name "omar," for example).
You log on to Outlook Web App, and then you try to open the shared mailbox.
In this scenario, the shared mailbox does not open, and you receive the following error message:
HTTP 400 - Bad Request
This issue occurs because Exchange considers a URL that contains the "/owa/oma" string a request for a service.
To resolve this issue, install the following update:
2961522 Update Rollup 7 for Exchange Server 2010 Service Pack 3
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.