Function insert_test_email_delivery_within_limit 

pub async fn insert_test_email_delivery_within_limit(
    conn: &mut PgConnection,
    user_id: Uuid,
    email_template_id: Uuid,
    subject: &str,
    content: &Value,
    placeholders: &Value,
    max_per_minute: i64,
) -> ModelResult<Option<Uuid>>

Queues a test send of an unsaved template body to the account’s own address, unless the account already queued max_per_minute in the last minute, in which case returns None. placeholders must hold every substitution: the sender derives none for a test send.