In that mail_client API post, I have given the example to load emails, but if you want to load only the new mails then you can use the date criteria to pick only for the current day. For example, modify the cursor of load_email procedure as follows:

CURSOR c_Inbox IS
  SELECT Msg_Number,
  Subject,
  Sender,
  Sender_Email,
  Sent_Date,
  Content_Type
FROM TABLE(Mail_Client.Get_Mail_Headers())
  where trunc(sent_date) = trunc(sysdate)
 ORDER BY Msg_Number DESC;

Also, you need to identify the unique column in the above header so that you can download only the new emails. Then you can schedule the procedure to run every hour to get the daily unique mails. You can check the following tutorials:

Run PL/SQL Job every hour example

Run PL/SQL job every day once