Function deliver_domain_static 

pub async fn deliver_domain_static(
    resolver: &TokioResolver,
    hostname: &str,
    domain: &str,
    messages: Vec<QueuedMessage>,
    pool: &PgPool,
    port: u16,
    max_per_conn: usize,
    event_sender: Option<&DeliveryEventSender>,
)

deliver messages to a single domain (used by concurrent workers)

port is the TCP port used to connect to every MX in the resolved set. Production wires 25; integration tests pass an ephemeral mock-server port to drive the full claim → MX-resolve → SMTP → mark_* lifecycle without a real MTA on the box.