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 firstname.lastname@example.org primary SMTP address and email@example.com secondary smtp address. John Doe can easily receive the message addressed both to firstname.lastname@example.org and email@example.com, but he can not send the message as firstname.lastname@example.org 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.