We are aware of some issues with emails to Yahoo addresses which are bouncing back.
We have taken this issue up with our upstream provider, who tells us:
We are currently experiencing an email issue with the Yahoo network and our team has identified the problem. Unfortunately, it would take time until they unblock or release the rate limit.
There is currently no estimated time to resolution available, however, we will post updates as soon as further information is made available
We apologise for any inconvenience this may have caused, and we truly appreciate your patience and understanding.