Originally Posted by ST_BWard
You can set up Internal Spammer Notification (Security | Advanced Settings | Abuse Detection) to be alerted when this does occur so you are able to take action and find the account that is compromised. Since the emails are spam you can enable Outbound SMTP blocking (Security | Antispam Administration) to prevent or minimize the amount of spam that is sent through your server.
If the spammer have a compromised account and are authentication with that account SmarterMail will accept the email and then relay the message.
These are a couple of ideas that may help. If you need further assistance you can open up a support ticket with SmarterTools and our support team will be glad to assist you.
This is a known issue and there are many complaints in the Smartertools forums. We have tried lots of options, but the spamming does not stop. Also, we have over 12000 email users and there are lots of users which are being compromised. As you can see it can be difficult to find each of them. The simple option that is required is to disallow sending of emails from null senders.