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.
ChooseFrom for Exchange 2007/2010/2013 application allows user to send message with any sender address (From: field), including his/her additional SMTP addresses.