Why is Yahoo greylisting e-mails?

[Greg]

For some reason, my father is receiving 451 - message temporarily deferred error messages when he tries to send from his Centurytel to my Yahoo mail account. The e-mail seems to get through eventually but takes 2 - 10 hours before Yahoo! will allow it go be delivered. Why is Yahoo doing this and how can I correct it?

[EyeNu-It]

Centurytel might be an uncommon email server. Then again 451 said an error occurred on the Server. But it says nothing of greylisting.
Lately its not uncommon for my account to get email some messages 1 to 3 days after it was sent.