Spam email is getting through all filters. Obvious spam emails are allowed through, being marked as whitelisted. Some emails may appear to be internal, although headers say otherwise.
This issue manifests when the sender or the receiver is whitelisted, and these whitelists have a higher priority than the filters, in the Filter Priority List. This can be caused by the IP Whitelist, Keyword Whitelist, Personal Whitelist, Email/Domain Whitelist, and Auto Whitelist.
In order to find the specific reason why the messages in your domain are whitelisted, please follow the steps detailed in the "Determining why an email was Whitelisted by MailEssentials" article.
Once you have determined which whitelist is allowing the spam email through, remove the rule in order to stop the undesired behavior.