Function format_dsn 

pub fn format_dsn(
    reporting_mta: &str,
    sender: &str,
    recipient: &str,
    error: &str,
    message_id: Option<&str>,
) -> String

Format a DSN (Delivery Status Notification) bounce message per RFC 3464. The returned message uses CRLF line endings (RFC 5322 §2.1) and multipart/report; report-type=delivery-status.