Exchange mailbox may have multiple SMTP addresses. For outgoing messages Exchange always uses the primary SMTP address. For example, John Doe has a mailbox with email@example.com primary SMTP address and firstname.lastname@example.org secondary SMTP address. John Doe can easily receive the message addressed both to email@example.com and firstname.lastname@example.org, but he can not send the message as email@example.com because this behaviour is by design in Exchange server. ChooseFromOWA for Exchange 2003 application allows Outlook Web Access users to send message with any of their own SMTP addresses as the From address.
||Free to try
||Windows 2003 Server, Microsoft Exchange 2003