mimeheader NAME_OF_RULE Header-Name =~ /pattern/modifiers
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 headers 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.