Consider a firm ABC running Exchange 2007. The Accounting department staff has their own mailboxes (user@company.com) and they are Outlook users. Their e-mail is handled through generic department distribution group (accounts@company.com), which ensures that all users receive each e-mail message. For internal mail they want to be able to send e-mail as each individual user, however when this user sends e-mail out of the office, the From address needs to be set to the address of the distribution group (accounts@company.com). This should be done automatically on Exchange server side.
Optionally, to send an external message personally, the user should set the message Sensitivity parameter to “Personal”, “Private” or “Confidential” .

Using SendAsGroup application would solve the problem.