"Send As " problem driving me nuts

[lowj]

I need to allow one of our offices to send external E-Mail on behalf of a distribution list. I have granted the users Send As permissions for the list. It works for users with External Email addresses who are already specified in the 'IMS properties\Delivery Restrictions\Accept messages from' dialogue box, but doesn't work for users who are not. If i put the users in here then they can send externally on behalf of the distribution list but they can also send externally without specifying the distribution list address in the FROM box. I have sent test messages to my hotmail account and this appears in the FROM field - GalwayCounter1 <IMCEAEX-_O=FBD+20INSURANCE+20PLC_OU=HEAD+20OFFICE_CN=RECIPIENTS_CN=GALWAYCOUNTER@FBD.ie> . I know this address does not have an external address so where does it get the GalwayCounter1@FBD.ie address from? I can also reply to this address and it is recieved by the GalwayCounter user. I can't allow all our branch office users external email but need to grant them the Send As permissions. Any ideas, thanks in advance.