Your installation of Simple Accounting can be modified to automatically accept transactions from other computer systems–even systems not using Simple Accounting via the MAPI Messaging System built into Windows. Thus, virtually any application that can send a transaction via e-mail (or any person who can type a message in a specific format) can send transactions automatically to your installation of Simple Accounting.
This process is currently one-way, ie. you can receive transactions from other systems, but you cannot automatically send Simple Accounting transactions to another system (except for another Simple Accounting system of course!)