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Duplicates
----------
If you are a member of a couple of mailing list, you will sometime
receive two copies of the same mail. This can be quite annoying, so
`nnmail' checks for and treats any duplicates it might find. To do
this, it keeps a cache of old `Message-ID's -
`nnmail-message-id-cache-file', which is `~/.nnmail-cache' by default.
The approximate maximum number of `Message-ID's stored there is
controlled by the `nnmail-message-id-cache-length' variable, which is
1000 by default. (So 1000 `Message-ID's will be stored.) If all this
sounds scary to you, you can set `nnmail-treat-duplicates' to `warn'
(which is what it is by default), and `nnmail' won't delete duplicate
mails. Instead it will generate a brand new `Message-ID' for the mail
and insert a warning into the head of the mail saying that it thinks
that this is a duplicate of a different message.
This variable can also be a function. If that's the case, the
function will be called from a buffer narrowed to the message in
question with the `Message-ID' as a parameter. The function must
return either `nil', `warn', or `delete'.
You can turn this feature off completely by setting the variable to
`nil'.
If you want all the duplicate mails to be put into a special
"duplicates" group, you could do that using the normal mail split
methods:
(setq nnmail-split-fancy
'(| ;; Messages duplicates go to a separate group.
("gnus-warning" "duplication of message" "duplicate")
;; Message from daemons, postmaster, and the like to another.
(any mail "mail.misc")
;; Other rules.
[ ... ] ))
Or something like:
(setq nnmail-split-methods
'(("duplicates" "^Gnus-Warning:")
;; Other rules.
[...]))
Here's a neat feature: If you know that the recipient reads her mail
with Gnus, and that she has `nnmail-treat-duplicates' set to `delete',
you can send her as many insults as you like, just by using a
`Message-ID' of a mail that you know that she's already received.
Think of all the fun! She'll never see any of it! Whee!