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In general, you should be fine (although using 1.0a5 rather than 1.0a6 might be advisable). Why the warning about 1.0a6?

There's a bug in it that means that you need to restart SpamBayes after every time you change your options via the web interface. Not serious, just annoying, but it could be worth using 1.0a5 and then getting 1.0a7/1.0b1 when it comes out. There has been talk on -dev about putting 1.0a7 out soon to address this, although I'm not sure when we'll find a chance to do it. =Tony Meyer