G Mail alias

Level 2

I have an alias set up in Gmail which has worked well for some time but it ceased working recently and I am unable to fix the problem. I get this error msg : (You're sending this from a different address or alias using the 'Send mail as' feature. The settings for your 'Send mail as' account are misconfigured or out of date. Check those settings and try resending.)

I have tried all variations of [port 25 & 587] [mail.tpg.com.au & smtp.tpg.com.au] [with and without TLS] [ editing and delete/reinstall alias]

Is this related to our recent conection to the NBN?

Any help is appreciated