Yes I’m afraid iMail (Part of UK Mail) did encounter some issues early December (see here for more details).
We were informed by iMail that those items would still have been dispatched but their server was returning an incorrect response. When we received the error response from iMail we would have automatically re-credited the fee.
This problem stemmed from a server update that was going on at iMail at the time. I am sorry for the inconvenience it has caused I am still waiting for a full report from iMail as to the scope of the problem. We were informed that any items with this response would have been dispatched as normal, although if there are cases where you believe this didn’t happen we will take this up with iMail.