Exchange 2000 mailbox may have multiple SMTP addresses. For outgoing messages Exchange 2000 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 2000 server.
ChooseFrom for Exchange 2000 application allows to send message with any sender address (From: field) , including user's additional SMTP addresses. Application is a transport event sink.