It is the other way around. Applications that want to send messages using your installed email application can use simple MAPI to send the details to the email application. That will open a new mail compose window in eM Client (or whichever application you use to send email) with the data provided.
Yes, as I said, this is usually used by database type applications, like accounting apps, to send email. So it sends the recipient address, the subject, and the body text via simple MAPI. Then you see a compose window from the email application open with all that data inserted and all you need to do is click on Send.