ChangeSender adds one important piece of functionality to Microsoft Outlook when used with Exchange Server: It allows you to choose the sender address your emails are sent from on a mail-by-mail basis.
Without ChangeSender, Exchange always sends out emails on your default email address fixed in the ActiveDirectory even when answering emails received on one of your additional email addresses.
Also, Exchange does not allow sharing the same email address (i.e. department-wide or company-wide email addresses) between users. ChangeSender solves both problems by adding a configurable "send as" selection box to your Outlook email form.