Global system mail rule sometimes doesn't work

Hi everybody,I’ve got a very strange problem concerning the system mail rules.

I’ve created a mail rule which should filter mails containing a specific phrase in the subject.

First, the rule seems to work fine, but sometimes, mails, which should be filtered, pass. I think that about 95 percent of the specific mails are filtered but not the other 5 percent.

I can not explain why. When I check the subject field of the (passed) mail document, the demanded phrase is contained. When I forward the mail, so it has to pass the rule a second time, it will be filtered.

Does anybody has an idea what the problem could be? Is there a way to debug if a mail passed a rule respectively if the rule was executed on a mail?

Thanks for your answers!

Wish you a Merry Christmas!

Regards,

Christian