Benjamin, likely something in the content of these messages is hanging up the spam check process, and leading to a stalled spool. I've sent you a PM with the custom build download link so you can give this a try - as it should resolve those issues. This fix will also be included in our next minor release.
That aside, stalled spool issues involving "Spam Check" status can vary in cause, and in the behavior they present. As such, we recommend submitting a support ticket to allow us to investigate further. If its determined the issue is related to a bug, we'll refund the support ticket back to your account. Thanks for reading! Have a great day!