I'm getting similar pop up error message when I open Outlook (Office 2016) to view my TPG emails. This started today.
The pop up warning states; "The server you are connected to is using a security certificate that cannot be verified. The target principal name is incorrect." When you view the security certificate it indicates it is issued to "mail.wondercom.com.au", not TPG, and expired 7/05/2017.
According to web searches, "Wondercom" appears to be an old subsidiary business of TPG.
If that is the case, why is TPG all of a sudden today, using a subsidiary company's expired security certificate on TPG's mail server????
This needs to get fixed now.