"Client does not have permissions to send as this sender"

This is an error being generated by your Microsoft Exchange server.  The error indicates that the primary email address and the Return email address are different and that the primary does not have permissions to 'send as' the return address.  

To take a closer look at this, head to Company → System Setup → Email.  You'll see the primary email address for all of ESC's email functions entered in the Login User field.  Under the Addresses sub-tab, you'll see the three basic Return addresses that ESC uses for its email functions.  However, it is possible to manually enter a 'From' or Return address in the Email screen when sending a message.  

The Fix

In Microsoft Exchange, the primary email address you found in the System Setup → Email tab needs to be given the proper permissions for sending as another email address.  

Unfortunately, our staff is not able to address issues in Microsoft Exchange, but Microsoft support can.  More information on this issue can be found in their Knowledge Base here: https://support.microsoft.com/en-us/kb/2572646.