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### 3.2 Mail Headers

`message-required-mail-headers`
  See News Headers, for the syntax of this variable. It is `(From Subject Date (optional . In-Reply-To) Message-ID (optional . User-Agent))` by default.
`message-ignored-mail-headers`
  Regexp of headers to be removed before mailing. The default is
  ‘`^[GF]cc:\\|^Resent-Fcc:\\|^Xref:\\|^X-Draft-From:\\|
  ^X-Gnus-Agent-Meta-Information:`’.
`message-default-mail-headers`
  This string is inserted at the end of the headers in all message buffers that are initialized as mail.
`message-generate-hashcash`
  Variable that indicates whether ‘`X-Hashcash`’ headers should be computed for the message. See Hashcash. If `opportunistic`, only generate the headers when it doesn't lead to the user having to wait.