1. Create a new Distribution Group using Exchange Management Console.
2. Add members to the group
3. Group Name > Properties > Mail Flow Settings > Message Delivery Restrictions. Make sure “Require that all senders are authenticated” checkbox is cleared. This allows to receive emails from external senders.
4. Open Exchange Management Shell and run:
   Add-ADPermission GroupName -ExtendedRights Send-As -user user.name
   GroupName – Name (not Display Name) of the group
   UserName – Domain logon of the user who will have Send As permission.

5. Repeat step 4 for all required users.
6. Wait for Exchange/Domain replication to propagate permissions. Can take up to 2 hours.
7. If you can’t wait 2 hours try following
   • Restart Microsoft Exchange Information Store service on the Exchange Server. All users will be temporary disconnected and Outlook users may need to restart the Outlook to reconnect.
   • Update offline address book:
     • On Exchange Server: Exchange Management Console > Organization Configuration > Mailbox > Olline Address Book > [address book name] > update
     • On Client Machine: Outlook 2010 >  Send / Receive Tab > Send/Receive Groups > Download Address Book > OK

MS Exchange 2007
MS Small Business Server 2008 (SBS 2008)