To Andrew Crystall

"... and in the mean time badly configured greylisting software - something like 60% of the time I encounter it - is dropping legitimate mail from legitimate senders because it's spam."

In that case you are almost certinly not running a properly setup mail server - it actually takes effort to set up greylisting to drop legitimate mail (or rather mail from compliant senders). But look on the positive side, at least with greylisting you should know that your message didn't get through (unless, again your own systems are broken) instead of it having been silently thrown away by spam filters !