You can't know that mails have been sent from your web application. It is the responsibility of the mail server to process emails, not the web server - this means that your code will use mail code, which uses the mail server (e.g. exchange) to actually send out the mails.
What you could do, is check the mail server to see what's been sent out on this email address. It's not perfect, because you may have a user who sends mails on this account, so it will only tell you that mail was sent - not that this mail was sent by your code.
If you need this functionality, you'll need to write this information out at some point - e.g. when the code requests the send mail.