I'm getting a similar error message
Task 'mail.tpg.com.au - Sending' reported error (0x800CCC6F) : 'Your outgoing (SMTP) e-mail server has reported an internal error. If you continue to receive this message, contact your server administrator or Internet service provider (ISP). The server responded: 554 5.7.1 [TPG-S02] [14F1Vn6H028465] Message rejected as it appears to be spam. Use SMTP AUTH, PostOffice, or contact the Postmaster Team <email@example.com>'
My outgoing code is aleady set at the number suggested in an earlier solution. I am on Outlook.
Welcome to the Community!
We've created an article that may help you resolve this issue. Check this link: Cannot Send Email Rejected by the Server/Banned from using SMTP
If you are still experiencing issues with the service, please try sending emails via TPG Post Office if it's going to work.
Send me a PM with your TPG username or customer ID number along with the problematic email address.