-D maxlen idcache
Formail will detect if the Message-ID of the current message has already been seen using an idcache file of approximately maxlen size. If not splitting, it
will return success if a duplicate has been found. If splitting, it will not output duplicate messages. If used in conjunction with -r, formail will look at
the mail address of the envelope sender instead at the Message-ID.
Don't you mean:
the mail address of the envelope sender instead **OF** the Message-ID.