[ale] Were we hacked?

[ChangingLINKS.com]

Yesterday, I started getting bounced emails that I did not send out. Can you 
tell me if the origin of the email was my client/local machine or remote 
server?
I am thinking that I may have gotten a "emailing virus" hacked into my local 
machine, or that my server was hacked. I got several reports that emails sent 
to me two weeks ago bounced (which may be related) for the first time. I 
understand that there are also ways that someone could send email via my 
server without hacking it - I just need to figure out what is happening 
before I get blamed for it.