How to split sending email

Hello,
I have implemented a mailing system to permit all the users of my site to send how much emails they want, but my host provider has set the SMTP Server to a limit of 3000 email per day.
 
I need a procedure to split the sending emails, but the difficulty is in the fact that I can't list the emails to send because they may come from different users in different time.
 
I think I need to insert ALL the email in a "Temporary List" and  take from it the sending emails, when the server has sent one mail I can delete it from the Temporary List.
 
But... how can I receive from server the sending notification?
 
Is there another way to procede?
 
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