Exchange 2007 Sending with Second Email Address (trying to use Send As)

Question

I support a company who just bought another company.  I'm trying to bring a couple of users' old email addresses in to Exchange and have it work with their old and new addresses.  They can receive mail under old and new accounts just fine.  Sending
under the old address is the problem.  They still need to send under the old address for existing contracts.
My first attempt was to create a distribution group, give it the user's old email address, and then give the user "send as" rights for the group using the Exchange Management Shell.  I ended up with the same error I'll detail below.
My next (and current) attempt was to set up a shadow user in AD with the old email address and then give the user "send as" rights to the shadow mailbox.  I assigned the "send as" permission by right-clicking on the mailbox and choosing
"Manage Send As Permission" and then adding the real user to the list.  In Outlook, I clicked From and then choose the shadow user account from the Global Address Book.  This added the address to the From drop down box.
This works, once...  Every time I restart the Information Store service, I can get one email to send out successfully.  Here is the error I get on every other attempt.  Notice it says "send on behalf" in the error.  Maybe I'm not
setting it up correctly in Outlook?  I didn't make Outlook screenshots but you'll find my AD and Exchange setup screenshots below.
Thanks for the help.  
(also posted at http://forums.msexchange.org/m_1800602162/mpage_1/key_/tm.htm#1800602162)
***ERROR START***
Delivery has failed to these recipients or distribution lists:
Tripp Beasley
You are not allowed to send this message because you are trying to send on behalf of another sender without permission to do so. Please verify that you are sending on behalf of the correct sender, or ask your system administrator to help you get the required
permission. 
Diagnostic information for administrators:
Generating server: 
[email protected]
#MSEXCH:MSExchangeIS:/DC=local/DC=XXXXXXXXXX:XXXXXXXXXX[578:0x000004DC:0x0000001D] #SMTP#
***ERROR END***

Answer 1

open the list of previous recipients and select the ones you need by holding the command button. then press "add to address book".