If someone is complaining that they received an email from your domain that you didn't send, it is usually NOT an indication that your virtual server was hacked. The problem is that a sender can put whatever they please as the return address in their email messages, regardless of where the sender is really sending the message from. But why would anyone put your domain name in their return address? Here are the two most common reasons this happens:
A virus sent the email. This is the most common reason. Many viruses that are spread today hijack infected computers and silently propagate themselves to other uses by emailing themselves through the infected computer. But, just because someone received an email message from your domain does not mean that you have a virus. Of course, the first thing you'll want to do is get a virus scanner and check your system to eliminate the possibility of there being a virus on your system. If no virus is found, then the most likely cause is that someone that has your email address has a virus. Most newer viruses today find random addresses from the email archives of the infected computer and use them as the return address when it proprogates itself by email. Virus writers do this because it makes it harder to identify where the virus originates from.
A spammer sent the email. Spammers are users that send unsolicited email advertisements. They almost always use someone else's domain name in the return address of their message in an attempt to hide where the spam is actually coming from.
In both of the above cases, there is nothing we can do from the server side to stop the messages from being sent because the messages are not being sent from or through our servers. If you want to take action against the sender you can usually get the IP address of the sender from the raw email header and report the abuse to their ISP. You may also be able to take legal action, but this is something you'd have to consult with a legal advisor about. Keep in mind that in the case of a virus, that the sender usually has no idea that the messages are being sent from their computer.