We're aware of this problem as it has been reported by multiple users, and have been trying to address it.

As a side-note, please note that Bayesian filter technology was introduced in SpamFilter 7 years ago, but is now a bit old, and spammers can easily work around it. For this reason, this filter is the last to be used to check for spam, as the other filters are usually more effective. With large installations we actually recommend disabling this filter, both to gain in performance and in accuracy, as again, spamers have learned to write emails in such a way as to bypass the Bayesian filters quite easily.