At times there may be accounts that the DST does not process properly typically because of an invalid email address/formatting issue.

The error thrown is as follows:
Invalid entries were found while processing active directory objects for synchronization
Extended Properties:
Error Type: InvalidUsersOrAddresses
Invalid entries:
Non-existent users:
User Guid:5b67d96a-6bb5-4ac4-ae01-da1692381c44
(This error might be caused by inconsistencies in the data. The service will attempt to resolve the problem automatically.)
Timestamp: 10/1/2010 6:27:02 PM (UTC)
Process Name: C:\Program Files\Microsoft Directory Synchronization Tool\Microsoft.Ehs.DirSync.Dst.exe
Category: General
Priority: 2
Application Domain: Microsoft.Ehs.DirSync.Dst.exe
Process Id: PID
Win32 Thread Id: TID
Thread Name:

Unfortunately the data given is somewhat sparse in that you need to locate the actual user and info related to that GUID given.  The following dsquery filter can help you with obtaining that data to examine the properties that are synced.

For a GUID that looks like 5b67d96a-6bb5-4ac4-ae01-da1692381c44

dsquery * forestroot -filter "(&(objectClass=[person|group|publicFolder])(objectguid=\6a\d9\67*)) -attr displayname proxyaddresses

NOTE: the objectguid format starting with the end of the first sequence and using the slashes and then just wildcarding the rest with the * dsquery must get the GUID in that syntax.

The output should look something like the following which will show you the user name and email address(es) to allow you to see if there is possibly something wrong with user.

  displayname    proxyaddresses