I will add a somewhat less exact, but simpler answer. I would
reasonably trust the exim spool ID to be unique for each message handle
by exim, even in a high-traffic situation. (It might even be possible
that it is guaranteed to be unique, but I would reserve the right to
make such guarantees to PH10 [The author of exim, in case you werent
aware])
I would not trust the "Message-ID:" header to be so, or to even be in
correct format or to exist at all. (Although if it doesnt exist, I
beleive exim adds one, using its spool ID, followed by an @, and the
exim hosts FQDN..)
On Fri, 3 Nov 2000, Phil Pennock wrote:
> On 2000-11-03 at 13:32 +0000, The Flying Hamster gifted us with:
> > abusing other people's domains atm :/). One thought is to use the
> > exim msg ID as a unique key, which naturally brings up the question
> > "is it unique" (or suffciently so over a given time period).
>
> Read:
> <http://www.exim.org/exim-html-3.10/doc/html/spec_3.html#IDX33>
>
> Found by going:
> <http://www.exim.org/>
> -> "Documentation and FAQs" (top-left pane)
> -> "The Exim Specification" (main panel)
> -> "message: id" (concept index (bottom-left pane))
>
>
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