MIMEHeader - perform regexp tests against MIME headers
  loadplugin    Mail::SpamAssassin::Plugin::MIMEHeader
  mimeheader    NAME_OF_RULE    Content-Id =~ /foo/
This plugin allows regexp rules to be written against MIME headers
    in the message.
  
  - Specify a rule. "NAME_OF_RULE" is the
      name of the rule to be used,
      "Header-Name" is the name of the MIME
      header to check, and
      "/pattern/modifiers" is the Perl regular
      expression to match against this.
    
Note that in a message of multiple parts, each header will be
        checked against the pattern separately. In other words, if multiple
        parts have a 'Content-Type' header, each header's value will be tested
        individually as a separate string.
    Header names are considered case-insensitive.
    The header values are normally cleaned up a little; for
        example, whitespace around the newline character in "folded"
        headers will be replaced with a single space. Append
        ":raw" to the header name to retrieve
        the raw, undecoded value, including pristine whitespace, instead.
   
  - tflags NAME_OF_RULE
    range=x-y
 
  - Match only from specific MIME parts, indexed in the order they are parsed.
      Part 1 = main message headers. Part 2 = next part etc.
    
    
 range=1    (match only main headers, not any subparts)
 range=2-   (match any subparts, but not the main headers)
 range=-3   (match only first three parts, including main headers)
 range=2-3  (match only first two subparts)
    
   
  - tflags NAME_OF_RULE
    concat
 
  - Concatenate all headers from all mime parts (possible range applied) into
      a single string for matching. This allows matching headers across multiple
      parts with single regex. Normally pattern is tested individually for
      different mime parts.