There is a special set of expansion variables containing the headers of the message being processed. These variables have names beginning with `$header_' followed by the name of the header, terminated by a colon. For example,
The whole item, including the terminating colon, is replaced by the contents of the message header. If there is more than one header with the same name, their contents are concatenated, with a single newline character between them. The capitalization of the name following `$header_' is not significant. Because any printing character except colon may appear in the name of a message's header (this is a requirement of RFC 822, the document that describes the format of a mail message) curly brackets must not be used in this case, as they will be taken as part of the header name. Two shortcuts are allowed in naming header variables:
If the message does not contain a header of the given name, an empty string is substituted. Thus it is important to spell the names of headers correctly. Do not use `$header_Reply_to' when you really mean `$header_Reply-to'.
You can test for the presence or absence of a header by means of the `def' condition, which is described in section 32.
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