For a message to be split, it looks for the parent message in the
References or In-Reply-To header and then looks in the message id
cache file (given by the variable `nnmail-message-id-cache-file') to
For a message to be split, it looks for the parent message in the
References or In-Reply-To header and then looks in the message id
cache file (given by the variable `nnmail-message-id-cache-file') to