Function insert_email_delivery 

pub async fn insert_email_delivery(
    conn: &mut PgConnection,
    user_id: Uuid,
    email_template_id: Uuid,
) -> ModelResult<Uuid>

Inserts an email delivery; fails if the user or email template is soft-deleted.

For a template whose substitutions the sender cannot look up from the account, use insert_email_delivery_with_placeholders.