Something similar happened to me a few weeks ago, friends were sending emails which were not arriving.
I contacted BT Yahoo online help and received the following advice which has worked perfectly. Just why my friends had been changed to blocked I still do not know!
Please login to your e-mail account and set the “Spam Guard” is “ON”
from the Bulk folder. I request you to follow the steps given below to
check whether any e-mail address is blocked from your end:
1. Login to your Account using BT Yahoo! e-mail address.
2. Click on ‘Options’(right top corner).
3. Click on ‘Filters’.
4. Click on ‘Add’ button.
5. Select ‘does not contain’ from the ‘To/Cc header’ dropdown menu
6. Type your BT E-mail Address in the ‘To/Cc field’ .
7. From the ‘Move the message to:’ dropdown menu select ‘Trash’ folder.
8. Click on ‘Add Filter’.
If you are not getting the e-mail messages from particular e-mail
address, I request you to check whether the e-mail address has been blocked
by the steps given below:
1. Login to your account using the BT Yahoo! e-mail address.
2. Click on ‘Options’.
3. Click on ‘Block Addresses’
4. Check whether the e-mail address is available from which you are not
5. If so, select the e-mail address and click on ‘Remove Block’.