Check outgoing mail? spam?

[spuppy]

I started receiving a bunch of bounced emails to one of my email accounts.. Apparently some russian spammer is sending out spams using my email as the "reply to" address. So all the bounced spam comes back to me.

I am wondering if they are somehow using my server to send these mails. But I'm not sure how to check what emails are exactly outgoing from my smtp server. How can I check this?

Furthermore, how can I disable this service, or password protect it?

Thanks!