Multiple Email Addresses in Outlook

I have 2011 SBS running 2010 Exchange. I have two businesses and receive email at two different email addresses appearing in one Outlook. For example. Michael@abc.com and Michael@123.com will both reach my outlook.  Michael@123.com is my primary email. My Outlook shows both accounts in a drop down, however if I select the secondary and send the email,  I get this reply:

Delivery has failed to these recipients or groups:
You can't send a message on behalf of this user unless you have permission to do so. Please make sure you're sending on behalf of the correct sender, or request the necessary permission. If the problem continues, please contact your helpdesk.

Thanks 

Michael