It is important to understand how email flows to understand exactly where the breakdown occurred. When you send an email from Outlook, it gets placed in a queue on Exchange. Exchange 2010 will then start the process of finding and communicating with the other receiving party to deliver this message.
Setup a firewall rule that blocks outgoing port 25 except for the email server.
Your email system might have a password compromised – hence a spammer can use your email system to generate mass emails. This can also eventually put you on a blacklist. This is way more common then you think.
This reverse DNS setup is an excellent step to prevent SPAM in general, but it is an extra and often forgotten step for the IT consultant or IT support personnel to get done.