A self-assertion which has a positive effect on the delivery rate of
the apparent sender is unlikely to be believed, as any crook can make
that assertion, and many will.
It's not quite so clear to me.
A self-assertion on the part of a sender which if believed would have
positive effect on mail _apparently_ from that sender, well, mostly
agreed, but see below.
A self-assertion on the part of a sender which if believed would have
positive effect on mail _actually_ from that sender seems like a
clear-cut case, but it's not quite so simple. Such assertions are
unlikely to be accepted blindly, but they _can_ be of use in that they
inherit the claimer's reputation: a sender with a good reputation can
usefully make such claims.
A self-assertion which has a negative effect on the delivery rate of
the apparent sender of mail is likely to be believed, as there's no
real incentive for the domain owner to publish it, apart from
"because it's true". "I send no mail" is the obvious example of that.
I disagree that there's no incentive to apparent senders to publish
such assertions: it makes them less attractive forgery targets. If the
entity has a good reputation in non-email respects, this may be a
substantial benefit.
"I send no mail" is interesting in another way, as it (by definition)
needs to be transmitted out of band,
I disagree that it _by definition_ needs to be out-of-band; it could be
sent in-band perfectly well - it's prima facie false in that case, but
that's not the same thing. (Admittedly, this is hairsplitting.)
Many other assertions are transmitted in-band "This is a mime
message", "This was sent on this date", "this was sent by this
person", "this is an html message" and so on.
Interesting you mention "this was sent by this person", because that's
an example of a self-assertion that can improve delivery - many
recipients use per-apparent-sender whitelists. The major reason such
assertions aren't useless in practice, I think, is that the assertion's
effect is recipient-dependent.
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