A complicated way to tell we do not send mail

Some domains state they will never send mail:

w3c.org.                172800  IN      TXT     "v=spf1 -all"

But cedex.net says it in a complicated way:

cedex.net.              85967   IN      TXT     "v=spf1 mx -all"

And the only MX is...

cedex.net.              85947   IN      MX      0 127.0.1.50.

I got a spam(?) claiming to be from cedex.net:

Aug  1 16:39:35 eve postfix/policy-spf[30331]: SPF fail: smtp_comment=Please 
see 
http://spf.pobox.com/why.html?sender=sina%40cedex.net&ip=218.118.224.195&receiver=eve,
 header_comment=eve: domain of sina(_at_)cedex(_dot_)net does not designate 
218.118.224.195 as permitted sender

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A complicated way to tell we do not send mail, Stephane Bortzmeyer <= Re: A complicated way to tell we do not send mail, David Beveridge Re: A complicated way to tell we do not send mail, David Beveridge Re: A complicated way to tell we do not send mail, Michel Bouissou A complicated way to tell we do not send mail, David Beveridge