.option dkim_sign_headers smtp string&!! "see below"
If set, this option must expand to a colon-separated
list of header names.
-Headers with these names, or the absence or such a header, will be included
+Headers with these names, or the absence of such a header, will be included
in the message signature.
When unspecified, the header names listed in RFC4871 will be used,
whether or not each header is present in the message.