Function with_thread_headers 

pub fn with_thread_headers(
    raw: &[u8],
    in_reply_to: Option<&str>,
    references: &[String],
    broken: bool,
) -> Vec<u8> 

The message with its threading headers replaced: In-Reply-To and References (folded continuation lines included) are dropped, and the given ones written at the end of the header block, when there are any. This is what break-thread and link-threads write back; mutt does the same to the message itself (mutt_break_thread clears both, link_threads sets In-Reply-To to the parent’s id). The line ending of the original is kept.