We run industry standard spam filters which block 99% of all spam...
Please delete any spam yourself as we would charge to do that. You can add captcha boxes to your webforms to block spam if you want. (Eg if spam appears to come through your website)
You should also install/update antivirus software on your computer.
I get 100s of undelivereds back to my email account: It happens from time to time people spam using your address, but not neccessarily compromised account. its recommended to first run virus checking, then change your email password. To clarify it can happen without anyone getting "in" to your account... technically anyone can use anyones from address.