Exchange 2007 : SendAs Bug
I recently came across an issue where SendAs for a Resource (Shared) Mailbox appeared to be failing, despite specifically defined permissions granting this.The mailbox in question had several email addresses defined, and the SendAs function was using one of the secondary email addresses.
It would appear that it is impossible to SendAs using an email address other than the primary (reply) email address, or display name of any Exchange object.