30 May
2004
30 May
'04
10:56 p.m.
It doesn't require a complete reimplementation at all - it is a plugin in your mail client and a plugin in the ISP's MTA.
If it is a plugin to your mail client then it is even less use. Until the scheme reaches sufficient penetration (estimate of 10 years according to one of the authors) that you can require mail senders to use it, spammers will simply use clients that don't implement it.