Welcome to the Community and thanks for your post!
I'm sorry for the problems your Mum is having accessing her mailbox and that you believe the account has been deleted. Do you want to try these steps here first of all and let me know how you get on? - Why has my email address been suspended or deleted?
Have been through this and BT who have confirmed account deleted and can no longer be recovered. She was accessing through her desktop in the same way she has done for years (not via yahoo) so was never inactive for 90 days. This is why we wish to make a formal complaint.
If you could please let me know procedure for doing so I would be grateful.
Thanks for getting back to us so quickly.
If you would like to make a formal complaint you can do this by following our Our Customer Complaints Code. The Mod team are happy to look into the technical side of this case and double-check if the account can be recovered or not. I have sent you a private message that you can use to contact the Mod team.
Just a little update on this case for the Community.
We have been able to get your Mum access to her BT Mail inbox via BT.com and have been able to restore all emails going back to the day that the service was ceased. I'm really glad we have been able to get the email address and inbox back.
We have sent a few test emails back and forward to confirm that everything is working.
The only thing is that your Mum will need to set up her email client on her Mac again but I have provided a copy of the email setting.