Using a non-TPG email address. It has been working fine for months. Stopped working yesterday. Error message is "Cannot send mail - the server rejected one of the recipient's addresses. The message has been placed in your Outbox".
Using MacBook Pro, iPhone XR and iPhone 12 - the standard Apple Mail account.
Incoming Host name: mail.optusnet.com.au
Outgoing Host name: mail.tpg.com.au
Are these settings correct?