I've got this (again) on my primary BT account when replying to mail in Outlook.
I last raised this same problem 27/7.
That case was closed 28/7 after some actions by BT following which all was ok **edited**
but the error message is back again.
Just to add that this time I cannot forward mail whereas last time I could. Forwarded mail gets bounced back in the same way as replied mail.
Webmail works fine
Thanks for your posts and welcome back!
I'm sorry that you're receiving that bounce back message when trying to send emails from your client. I have sent you a private message with instructions on how you can send us over your details and we'll look into this for you. See: Private messages