At one stage i= was thought to represent different mail streams with different
reputation,
however this did not get any traction...
As far as I can recall, I don't think anyone but you had that theory.
If you want two streams, you use two d= domains.
On my system the i= tells how the mail was injected (submit, webmail,
whatever) and who the user was if it knows. It's fairly obvious if
you look at it, but it's not really of any use to anyone but me for
tracking down the occasional odd user behavior.
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