I can send an email from my Yahoo address (using Web-site) to my Optusnet address.
However I can't send any emails from my Optusnet address (using MS Outlook 2019) to any Yahoo addresses, whether mine or several friends who all have Yahoo addresses.

I don't have Yahoo account in Outlook, only my Optusnet account, so can't delete it.
I don't know why I would need Yahoo account in Outlook, when I don't have a gmail or other account in Outlook apart from Optusnet.