5.7.1 Client does not have permissions to send as this sender
This is what your Microsoft Exchange 2007 and or 2010 server may report when you try to send email through the Exchange server when permissions have been fouled up. The First this to check on is the “Manage Send As Permissions” under the user mailbox properties in the Exchange Management Console. Access the Exchange management console and select the user that is having the issue. Right Click, go to the send as permissions option.. now check that User NT AUTHORITY\SELF is listed.
If it is not listed here then you need to add it in. This allows the authenticated user to send as their “Self”. Now retest your connection an try send a piece of mail.