Exchange 2K not delivering

One user is trying to send an email to the following address :
london@wca-chartered-surveyors.co.uk

We can ping the address and get a reply, we can hotmail to the address, other people can send to the company, but we get the following NDR : Could not deliver the message in the time limit specified. Please retry or contact your administrator.

Help mep lease – they need to be able to send these emails – all other addresses work fine