Unable to Send On Behalf of Another User

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Outlook version
Outlook 2013 64 bit
Email Account
Exchange Server
my goal is to be able to send and receive email from a 2nd exchange account within Outlook 2013. I have a local Exchange 2012 Server and have attempted to do this two ways;

First, I added my account through EAC site to the 2nd account with 'Send As', 'Send on Behalf' and 'Full Access' permissions (overkill, I know). Whenever I address a new email using the 2nd email account as the sender I get an NDR from SYSADMIN saying I don't have permission to send the message on behalf of the specified user.

Second, I added the 2nd mailbox within Outlook (Advanced settings of Account Settings). It appears in the navigation menu and I can open email and respond to email directly sent to the 2nd account and it shows as coming from the 2nd account not me (that's good) but I still cannot create a new message using the 2nd account without the NDR permission message.

Your assistance is appreciated.

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