The vast majority of the spam that sender validating systems might block after they have been installed in most SMTP clients 5 or 10 years from now is rejected today at any organization that really cares about spam using any of various tactics including DNS blacklists (e.g. the CBL or the so called "dynamic IP" lists) and greylisting.
I'd say "back this up" but then we're once again having one of these unproductive spam discussions. Why don't we all do this on our personal favorite anti-spam list and revist the issue here only when there is a pertinent draft in last call?
One last remark:
block port 25
This is evil for several reasons:
1. blocking ports reduces the functionality of the internet
2. doing work per-packet rather than per-session is inefficient (especially considering that only a small fraction of all packets is email to begin with)
3. requiring others to clean up their act rather than do something yourself is ineffective
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