error message is this in the Event viewer:

Event Type: Error
Event Source: Add2Exchange
Event Category: None
Event ID: 2501
Date: 2/14/2011
Time: 4:37:54 AM
User: N/A
Computer: OUTLOOK-SYNC
Description:
A2ESP Could not connect to server: EXCH2007 to test for service: MSExchangeIS please check EXCH2007 for: 1) the Server is unavailable or shut down. 2) ZADD2EXCHANGE Account is not part of the local Admins group. 3) Some recent Microsoft updates required a reboot and it is still pending.

For further information/support, please visit: http://support.diditbetter.com/knowledgebase.aspx

Manage the Computer and select WMI and then select properties.  If the test fails even on local (exchange server) machine, if Add2Exchange is installed on another replication server, if it fails locally, it will also remotely.

Result
No synchronization until this is resolved by one of the solutions below

Analysis
A wmiprvse.exe process may leak memory when a WMI notification query is used heavily on a Windows Server 2008-based or Windows Vista-based computer

Severity3 : Active : Windows Server WMI hotfix is missing

• Root Cause Status: Inactive ---> Active
• Impact List: MailboxServer:exch2007
• Severity Level: Severity3
• Description:Windows Server WMI hotfix is missing
• Time Detected: 2/14/2011 4:11:54 AM EST -05:00
• Root Cause First Occurrence: 10/27/2010 4:41:57 PM EDT -04:00
• Root Cause ID: 958124_1:10
• Knowledge Base Article: http://support.microsoft.com/default.aspx?scid=kb;en-us;958124

Resolution:
Do the update if applicable and reboot
Or 
Turn off the Wmi test under Services close the Add2Exchange Console and restart the A2E Service. This resolution should be a temporary fix because doing this procedure makes A2E "assume" the store is up instead of actually explicitly testing for it.