services such as MesssageLabs, Postini etc. do have other potential benefits
what are they?
A relative lack of reliance on dumb blacklists, which make most spam filtering services drop all emails from whole countries or foreign ISPs, might be one of them
Filters just don't work anymore - well they do but with masses of false positives which is no good for a business which cannot afford to lose emails from new contacts.
Most spam today comes from bots (zombie PCs) i.e. legitimate senders. They are not on dynamic IPs either; they are sending via their ISPs (legitimate senders).