Spam bypassing gateway server

[Chris Kissinger]

Trying to figure out how some spam is totally bypassing the gateway mail
server and not being scanned by MailScanner. The gateway server is the only
MX record, mail sent from the regular mail server to a local domain still
goes out and is scanned by the gateway server then delivered.

On the ones getting through there's no MailScanner headers added, there's
also no headers from it hitting the gateway server at all. If it was a
timeout issue shouldn't it at least have the regular mail headers added?

Any ideas would be great.

Chris