Ok, so here it goes...
My domain has a few UPN domains associated with it. When an email account gets created, our Exchange 2010 server assigns 3 email addresses, each with the same username, and each ending in my domains.
On our Exchange, I have created a general user account called Invoices. I am trying to set permissions for some of my users to send as that Invoices account, but I constantly get a permission denied from sending as this user message. I have ensured that the email account is not hidden from the GAL and double and triple checked that the permissions were set correctly in Exchange. Waited several hours and I still get the message denied error.
What am I missing?