I have been having the same issues and found your forum looking for the answer. I just worked through the issue and wanted to let you all know what I did...
My symptoms: Trying to Send As another mailbox user for which the sending user had full access permissions. This was not an issue testing from my login which is a Domain Admin. Received all variations of no access to send as the specified user when testing from non-Domain Admin accounts.
My fix:
Exchange - User must have full access permission and send on behalf permission. Send As permission not required. (the send as was not needed for the domain admin account. Inheritance setting were the same between the Domain User and Domain Admin accounts.)
Outlook - Open Mail from Control Panel and delete the existing profile. Create a new profile named Outlook and add your primary account. Choose "Add another account" after your primary account has been configured and add the account for which you've given yourself/the user full access/send on behalf permissions. This worked for cached exchange mode.
Open Outlook. You'll see the second account listed twice. Close Outlook. Open Outlook. The second listing will have dropped off. Click the second account and start a new message. The From should already be set as the secondary account.
In addition you may configure Signatures for each account. The signature will change depending on which account you start the message from, or for whichever account you choose the message to be From once composing.
NOTE: Another little nugget I learned along the way is that the account/s you wish to send on behalf of MUST NOT be hidden from the Address Lists.
Hope this helps someone!
