Indeed, I'd argue that a selfhosted learning spamfilter will be far more attuned to your personal mail-regime than one that has to do 'well enough' for millions of users.
The latter will catch new spam tricks easier. But the former will be trained by you. For you. For your niche, using your lingo and so on.
To take an extreme example: let's imagine you are the ITperson for a doctors' practice aimed at people with sexual disfunctionality. One thing is for certain: using Google or Outlook for your mail is a disaster waiting to happen.
The latter will catch new spam tricks easier. But the former will be trained by you. For you. For your niche, using your lingo and so on.
To take an extreme example: let's imagine you are the ITperson for a doctors' practice aimed at people with sexual disfunctionality. One thing is for certain: using Google or Outlook for your mail is a disaster waiting to happen.