Thanks for raising this to us, are you using an email client or TPG post office? Can you post the error message on this thread.
Here is an article that you may find helpful, Cannot Send Email Rejected by the Server/Banned from using SMTP .
Let me know how it will go.
I am receiving emails but am unable to send them from firstname.lastname@example.org this has been happening for approx 10 hours. Can you please assist me?