error to assign permission bes setup

 was getting the same error as you while repeatedly knocking my head against my desk! Turns out it had nothing to do with permissions, the error is because you are not specifying the EXACT Distinguished Name for your BESAdmin account. (solution pulled from blackberryforums.com)

Here's how to determine the exact Distinguished Name

1. On your Exchange Server launch Active Directory Users & Computers
2. Select View | Advanced Features (Check advance features if it isn't already)
3. Go down the OU's until you locate your user (BESAdmin in my case)
4. Right-Click on your BESAdmin user and select "Properties"
5. Click on the "Attribute Editor" tab
6. Scroll down in this list of attributes until you find "distinguishedName"
7. Highlight, click view, and copy the entire Value.

Mine is "CN=BES Admin,CN=Users,DC=mydomain,DC=com"

8. Now go back to the Exchange Management Shell and re-enter the entire "Add-ADPermission" command, this time using the Distinguished Name you found in ADUC.

Again the command is:

Add-ADPermission -InheritedObjectType User -InheritanceType Descendents -ExtendedRights Send-As -User "BESAdmin" -Identity "CN=BES Admin,CN=Users,DC=mydomain,DC=com"

Worked for me and took care of my  "Send As" warning the BES setup (going for a second try on loading it up)