Function get_uid_next 

async fn get_uid_next(
    context: &Context,
    transport_id: u32,
    folder: &str,
) -> Result<u32>

uid_next is the next unique identifier value from the last time we fetched a folder See https://tools.ietf.org/html/rfc3501#section-2.3.1.1 This method returns the uid_next from the last time we fetched messages. We can compare this to the current uid_next to find out whether there are new messages and fetch from this value on to get all new messages.