How spammers get past spam filters

My boss pointed something intersting out to me.  A penis enlargement email that keeps slipping past my spam assasin and his outlook client based anti spam software.

Check out this source…

at l<kwriomldwis>east 3 I<kophixzbmfahpt>NCH<kukntyadvxfru>ES or ge<kjgdkhybxvmbj>t
y<kggudaubtbo>our mon<kxeytamotthnl>ey bac<kpkevvobnpdeccq>k!

They throw giberish tags in between all the words so the spam filters dont find the words and the mail client just ignores all of the nonsense tags.

Spam filters should parse out all HTML and just look at the text IMO.