Happy to correct you, mailwasher downloads headers and the first x lines (you define x number) which are viewed in plain text (if you have elected to view the preview pane) if you need to check content, this information is held in a temp location but the email still exists on the server, it is not downloaded into your email inbox, spam or other folder in your email program.
That will limit the amount of downloaded data. In the past I used a program that did similar things. Then Spammers started to add blank lines at the beginning of the message
so it was of no use anymore.
They now ad blank lines spell words like t.hi.s use html text that is 0pt size so that you see t hi s, the human eye and brain can cope with this but many spam filters can't. This shows that spam blocking is working as they try ever more devious methods to get arround spam filters.
The other feature that I like is the Origin of Spam filter, it checks mail not by content but from origin, IP address and this is very accurate, it also allows you to add new databases of spamming IPs, I have added the one from SpamHaus.org (sbl-xbl.spamhaus.org, this checks two databases at spamhaus.org) to the other two which come by default.
BUT:
From this limited information it/you can identify and mark for deletion. It is currently recognising close to 100% of all spam
It seems the way of spam changed again and it will pick up spam again. I will give mailwasher a try. I am happy with spampal, but if there is another, better program around... Why not use that 
Thanks for the heads up.
Peter
Every mail that came in to one account that managed to get into the spam lists generated 25 emails (this morning, about 75-100 total per day), all identified correctly as Origin Blacklisted or Possible Spam (based on either baysian filter or my teaching it), I can't remember when I last had a false positive and the time savings have been great.
David