I have tried several anti-spam clients for Outlook recently, here are a few
reviews.

I’ll start with SpamBayes,
the open-source Python add-in for Outlook.

First of all, I have to say I like the idea of an Outlook add-in written in
Python.  I am not a big fan of Python myself (here
is 
why), preferring
Groovy and Ruby, but this is a testament to the goodness of COM/.Net which gives
you a lot of flexibility on your language of choice.

My initial contact with SpamBayes was pretty good but unfortunately, the
honeymoon didn’t last long.  After a few weeks, I became annoyed by the
following shortcomings:

  • SpamBayes is slow.  I don’t know if it’s due to Python (or its
    executable compiler) or the code itself, but you can clearly see it
    processing a message.  While it’s usually not an issue for individual
    messages, you will feel your pain when you haven’t launched Outlook in
    several days and that SpamBayes suddenly finds itself in front of over one
    hundred messages to filter.
     
  • Even after a few weeks of training, a high number of emails (about
    twenty per day) still ended up in the "Junk suspects" folder while they are
    obviously spam that should have been detected by the Bayesian algorithm
    (meaning:  they are of reasonable size, in plain English text and
    contain quite a few keywords that should have made the filter take immediate
    notice).
     
  • But the number one reason that made me decide to give up is: 
    SpamBayes doesn’t have a white list.

First of all, I was quite put off by the attitude of the developers when I
asked for that feature.  The responses were typically along the lines "SpamBayes
doesn’t need a white list, it’s doing a great job already", "I’ve never needed
it" and "Why don’t you add the feature yourself?".

This is not the kind of response you get from commercial vendors typically,
but well, the software is free so there is not much I can do.

But the worst part of this shortage is that it shows that the Spambayes
authors don’t understand that a spam filter is simply useless without a white
list.

It took just a few days for me to realize that when I started exchanging
important emails with someone who tends to write very short, poorly-formatted
emails that the filter was absolutely incapable of training against. 
Despite all my efforts, this person’s emails regularly ended up in the "Junk
suspects" folder or, worse, in "Junk".

Another example a few days later: emails from a member of my family who are
tagged with several lines of self promotion / advertising at the bottom, which
the filter systematically interpreted as spam.

After a few weeks of use, I realized that I just didn’t trust my spam filter. 
I kept dreading that I would miss an important email and therefore, applied
extra caution when perusing my "Junk suspects" folder, which completely defeats
the purpose of such a tool.  Added to the fact that SpamBayes doesn’t offer
extra goodies such as statistics or challenge/response, it became clear to me it
was time to look for another option.

Next:  MailFrontier’s Matador (and after that, IHateSpam).