Delay Delivery not working


We have found that the Delay Delivery function does not seem to be working.

I have tested firstly by only delaying by 1/2 an hour, and by a day. Both messages are still sitting in my Sent Items as Pending. This has worked in the past, but i'm not sure when it has stopped working.

Is there something fundamental I'm missing, or could this be a bug?

With thanks