Unable to send as delegate

Hello all! I have a user on my office network using a Mac Mini running the latest version of Mavericks, lets call them user V. User V has Outlook 2011 installed and has several delegates configured on the account. User V is unable to send as/on behalf of these delegates. Other people in her department can send as the same exact delegates with out issue. The error message that user V receives is:

Error
HTTP error. The server cannot fufill the request.

Details
Mail could not be sent. The message has been moved to your Drafts folder.

Account name: "Delegates name"

Error code: -18500

I have tried rebuilding the account on user V's computer, reinstalling all of MS office and have worked with MS support over several phone calls over the last few weeks to no avail. I have configured user V's account on a different Mac Mini and did not encounter the issue there.

If anyone has any suggestions on where to go next it would be greatly appreciated!

Thanks!