Function retry_mark_sent 

pub async fn retry_mark_sent<F, Fut, E>(
    id: i64,
    retry_delays_ms: &[u64],
    mark_sent: F,
) -> Result<bool, E>
where
    F: FnMut(i64, usize) -> Fut,
    Fut: Future<Output = Result<bool, E>>,
    E: Display,

Retries a product-provided mark_sent operation after SMTP success.

This helper exists to narrow the duplicate-delivery window where SMTP has accepted a message but the database row still says Processing. The caller provides the actual persistence function so repositories, transactions, timestamps, and product errors stay in the product crate.

Errors

Returns the final persistence error after every configured mark-sent retry fails.