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 john.doe@comapnya.com primary SMTP address and john.doe@companyb.com secondary smtp address. John Doe can easily receive the message addressed both to john.doe@comapnya.com and john.doe@comapnyb.com, but he can not send the message as john.doe@comapnyb.com because this behaviour is by design in Exchange server.

ChooseFrom for Exchange 2007/2010 application allows user to send message with any sender address (From: field), including his/her additional SMTP addresses.