Search for "send as" or "revoked" here BlackBerry Technical Solution Center
. RIM has a very detailed article about this issue.
If you're users cannot send when moved to a particular OU then the Send As is not applied to that OU.
By the way, you don't need to secify who you are sending as. It works better if you do the following:
1. Log into a WORKSTATION as BESAdmin
2. Create a MAPI profile for a user that cannot send
3. Create an email in Outlook and try to send it
If you get a message saying that you do not have permission send on behalf of the specified user then you have Send As.
One thing though, I have seen this work even permissions are still the issue. It's rare but it has happened.
Look at RIM's KB, it also has a tool to set Send As for you