You are posting on a two year old thread.
On the "old" BTMail the domains were blocked in the method described above ie @gmail.
On the "new" BTMail the domains are blocked with out the @ symbol. ie gmail.
I can see the blocked senders but how do I unblock those with the domain I need to keep, please>
Your post is not very clear.
Do you want to unblock a domain or do you want to keep the domain blocked but allow some emails from that domain through?
Blocking an email address does not block the domain.
If you block a domain it will block all emails from that domain.
If you want to allow a specific email address from a blocked domain you need to enter the email address into your "Safe Senders" list.
To unblock an email address just select it and click delete as already described in message 2.