I am constantly receiving email bounce notifications. The bounce notifications are sent to random addresses on my domain, such as BAKJSGKAG@mydomain.com. None of these are real accounts I have set up, but I know that if I receive mail it always goes somewhere.

What can I do about this? Is it a problem on my end w/ the software I have installed or is it a problem with my host? I am worried that a spammer or some other person is sending mail from my domain.

If it matters, I have php mailer on my site, and occasionally some of the more verbose return emails will mention it.