Possibly a further complication along the lines of this, @Badger9:
https://community.talktalk.co.uk/t5/Email/Important-information-about-changes-to-your-TalkTalk-Email...
If you are a current Talktalk broadband customer, your legacy email addresses need to be linked to the Talktalk account.
Are you seeing a banner about Everymail when you try to send?
If you are no longer a Talktalk/ ex Shell customer, you will need to subscribe to Everymail.
https://community.talktalk.co.uk/t5/Articles/Email-Transition-FAQs/ta-p/3114888
Gliwmaeden2, a fellow customer.
