From ::mime::copymessageaux

    if {[string compare $state(version) ""]} {
        puts $channel "MIME-Version: $state(version)"
    }
    foreach lower $state(lowerL) mixed $state(mixedL) {
        foreach value $header($lower) {
            puts $channel "$mixed: $value"
        }
    }

From ::mime::buildmessageaux

    if {[string compare $state(version) ""]} {
        append result "MIME-Version: $state(version)\r\n"
    }
    foreach lower $state(lowerL) mixed $state(mixedL) {
        foreach value $header($lower) {
            append result "$mixed: $value\r\n"
        }
    }