hey team, I too have this error, and although theres a lot of backslapping over the issue being fixed by changing passwords, I dont see how that can be.
I can email other TPG emails without issue from OUTLOOK 2010/Windows 10, and yet everyone else returns that error
Server error: '550 5.7.1 < >... Relaying denied. IP name possibly forged
and i can comfortably send email from the TPG Webmail/postoffice - no issues
surely this must be an issue with the mail server?