I am sorry to hear that you are both having this issue. However, in most cases this may need to be looked into by your hosting provider directly.
Do you receive any error responses after attempting to send emails from webmail? If so, can you provide me with the output of one of those "Mail delivery failed" messages? If no error messages are returned, I would suggest contacting your hosting provider so that they can access the server directly and take a closer look.
Unfortunately the "retry time not reached" error can be caused for numerous factors. Does this only occur when sending to Yahoo users? It is most likely that other emails from you or other users on the same server are being differed by Yahoo.com. It may also be possible that the messages will eventually be delivered after some time. Would it be possible to send us the full error message? Also, have you confirmed that there are no blocks put on the server directly? You can also inquire about this with your hosting provider directly.
Please let us know if you have any further questions or concerns.