From their "network status" page:
Some customers will either be unable to connect to their mailboxes or connectivity may be intermittent. Please note all customers will still be able to send mail, the current issue is only affecting some mailboxes for inbound mail.
Our engineering team is on site to attend to the affected machines and we are now aware that the affected mailboxes are currently being restored and the mail service will return to normal during the remainder of today. This will take a little time and we ask affected customers to be patient while we bring the remaining servers back online.
[ISP] sincerely apologies for this problem and the inconvenience it is causing. We are working hard to resolve this issue as quickly as possible.
I may need to unbounce myself at Yahoo later.