If you have found unknown source of emails sending through your mail relay account, make sure those emails were sent from your IP address.
If the emails have blank from email addresses, then those emails were sent by NDR (Non Delivery Report) and you must use our mail relay as smarthost(mail gateway) for your exchange server. Please click this link to know what is NDR. You can turn off NDR to avoid such email sendings. Click the following links to know how to turn off NDR on Exchange 2000 and 2003 .
If the emails were from your IP address. It is very possible that you must use our mail relay server as email gateway (smarthost) for exchange server. In this case, we are almost certain that one of the PCs that use your exchange server to send emails is infected with viruses.
To find out which PC in your network is infected, you can go to emails queued on your exchange server and inpect the emails. Try to check the email's headers to find out the source of the email, it should display the IP of the PC where the email was originated. You should do virus scan on the PC to clean the viruses.