variant_sigs/:
  this subdirectory contains a number of tests testing what happens
  if the signature is constructed differently than normal. It also tests
  signatures that are invalid according to the spec

headers/:
  test0.txt: multiple received headers
  test1.txt: rearrange order of headers staying below DKIM-Signature
  test2.txt: rearrange order of headers above & below DKIM-Signature
  test3.txt: header with a very long line (many recipients listed in To: header)
       (fails 'cause Postfix rewrites the To: header)
  test4.txt: large header (281 received lines)
       (fails with "too many hops")
  test5.txt: invalid header (line without a colon)
       (fails 'cause Postfix cleans it up)

TODO:
  - invalid From: or Sender: headers

TODO:
  - canonicalization tests
    ... make a bunch of messages
	    - different spacing in header, body
		- line breaks
		- tabs
		- folded headers
		- blank lines at end of body
		- missing body
	... then test these against each possible canonicalization combination

TODO:
  - multiple valid DKIM signatures
    (two signatures generated from same content, with different parameters)
  - daisy-chained DKIM signatures
    (signature generated from an already-signed-message)
  - multiple DKIM signatures, only one of which is valid
    (play "find the valid signature")