I've got an Exchange 2003 server and have users with Outlook 2003 and Outlook 2010. I've got several users who need to send email using the From address of a group.
The users (and a test account) have been setup with SendAs enabled. When logged onto the test account, I've forced a download of the global address list.
When I try to send an email with From specified as the group email address specified, I get the error, "You do not have permission to send to this recipient. For assistance, contact your system administrator".
I've setup an administrative-level account to do this successfully, but am obviously missing something with user-level accounts.