Outlook Dialogue box pop- up stating “Trying to Connect to Microsoft Exchange Server.” You may get a MAPI_E_LOGON_FAILED
Windows Server 2008 GC’s have a limitation of 50 concurrent NSPI connections per user.
Applications such as Outlook 2007, Blackberry Enterprise Server, or any application that will create multiple NSPI connections to a DC/GC can experience these symptoms.
To Resolve this issue, modify the following registry entry.
Click Start, click Run, type regedit, and then click OK.
Locate and then click the following registry key:
On the Edit menu, point to New, and then click Key.
Type Parameters, and then press ENTER.
Click the Parameters key.
On the Edit menu, point to New, and then click DWORD Value.
Type NSPI max sessions per user, and then press ENTER.
Double-click NSPI max sessions per user, type the number of the NSPI connections required, and then click OK.
Note: While the upper limit of this setting is 0xffffffff (or 4294967295), configuring the server with a value larger than the default will consume additional memory on the server – one new page per connection. If the value is set too high and too many connections are created for each user application instance, the server will run low on memory or become completely unresponsive. The lower default NSPI connection limit in Windows Server 2008 was based on customer experience, where previous operating systems would allow themselves to be overwhelmed by third party products in what amounts to a denial of service attack. Use a common sense approach to increasing beyond the default value; for example, start with decimal 250 (hex 0x000000FA), then test to see the memory overhead created and that the errors have ceased. Your long-term solution must be to contact the vendor of your NSPI product and ask them to change their behavior; the registry value is only as a workaround to provide error relief.
For more information, see the following article from the Microsoft Knowledge Base online: