exchange mailbox may have multiple smtp addresses. for outgoing messages exchange server always uses the primary smtp address. for example, john doe has a mailbox with john.doe@companya.com primary smtp address and john.doe@companyb.com secondary smtp address. john doe can easily receive the message addressed both to john.doe@companya.com and john.doe@companyb.com, but he can not send the message as john.doe@companyb.com because this behaviour is