Yahoo Mail Delivery Problems

We're seeing our deferred queues fill up with mail to Yahoo. Some mail
appears to be delivering, but many are not. Is anyone else experiencing
problems delivering mail to Yahoo? The Internet suggests that this error
has been a result of Yahoo stability in the past.

The following message is coming back:

host[] said: 451 4.3.2 Internal error

reading data (in reply to MAIL FROM command)
lost connection with[] while sending