When I wanted to create a test user for SCOM 2007 to monitor Exchange, I constantly ran into the error (whenever I ran the script
new-TestCasconnectivityUser.ps1):

CreateTestUser : Mailbox could not be created. Verify that OU ‘Users’ exists and that password meets complexity requirements.

I checked google, and all solutions pointed to the fact that there was more than one users OU.
When I edited the script and replaced the variable $OrganizationalUnit with a different OU, I still received the same error.

The solution came when I created a new OU in AD and edited the script again, now I changed the $OrganizationalUnit parameter to contain the newly created OU. After that the script ran perfectly.