Undelivered Mail Returned to Sender - Sending to BT email addresses

Undelivered Mail Returned to Sender - Sending to BT email addresses

Zoho mail is becoming unusable, I'm having to use my gmail address whenever email a BT email address.

I first get "Mail Delivery Status Notification (Delay)" and then a few days later I get "Undelivered Mail Returned to Sender"

Are other people experiencing this problem and have you found a solution?