From their "network status" page:
Following the malfunction of one of our mailservers, email service to some customers has been disrupted over the weekend and during part of Monday.
Service was restored to the affected server this morning (Monday 31st) at approximately 8am. After this, mailboxes on the server were gradually opened as messages recovered from the defective server were copied into mailboxes. As each mailbox was created login to the system once again became possible for that customer.
However, while this process was going on it was not possible to also deliver new email to these mailboxes. As timely email delivery is a priority for our customers, we have taken the decision to suspend the mailbox copying process and are creating blank mailboxes for all customers who don’t yet have a mailbox on the new server, so that they can login to their accounts and receive any email waiting for delivery to them.
Delivery of the waiting email has also commenced and the entire mailbox creation process and delivery of the waiting messages will be complete this evening. Every customer on the affected server will then be able to login and collect new email.
Work on the system is still continuing and further updates will be posted here in due course.
Once again, [ISP] sincerely apologises for these problems, which were caused by the failure of a critical piece of hardware in their server network.