I raised the question and got answer from moderator PeterW but I lost track of my link with him.
Already a week but when I tried to send mails - the result is UNDELIVERABLE.
This common problem I have found many posts in community forum and they need the Postmaster to unblock.
Cannot Send Email Rejected by the Server/Banned from using SMTP
If you can't send email using your Email Client but can send through our Postoffice, chances are your email address have been blocked from using our SMTP server.
You may get the following error message when sending email.
For MAC: Rejected by the SMTP Server
For Windows: Banned from using SMTP
Our SMTP Server automatically blocks and Postmaster can manually block email address from sending email using SMTP for security purposes.
Most common issue is when an email address is been compromised and being use for the following.
TPG SMTP servers detected that the email address is sending spam emails, virus or malware.
TPG SMTP server has detected that your account is sending message to a large number of recipients; Unsolicited bulk email (UBE) / Unsolicited commercial email (UCE).
TPG SMTP Server has detected that your email has been used with a foreign IP Address “overseas”.
You can follow the simple steps below to fix this issue:
Change you TPG account password online: https://www.tpg.com.au/home/myaccount
After changing the password. Wait for 15-30 minutes before sending an email.
Hopefully this helps you.