After creating the universal distribution group in active directory with the wanted ‘Send As’ address, I could not get outlook to allow me to send the email impersonating the distribution list email address. Running the following in the Exchange Server Powershell did the trick:
Set-DistributionGroup -Identity “<distribution list name>” -GrantSendOnBehalfTo “<primary email mailbox>”
PITA! But I guess they have their reasons. Hopefully the next version will facilitate this option.