I have already checked with our ISP (Fasthosts) and they have confirmed that there are no blocks or flags on the account.
The settings we have for the servers in Outlook 2010 are both the same and the tick box is checked to use the same settings for the outgoing server as the incoming server.
Is it possible that mail from our office is 'passing through' a BT open world server as they supply our broadband and Hub and it is this server that is blocking the mail.
The error message says Server error: '554 < host81-149-140-118 .in- addr.btopenworld.com [81 .149. 140.118]>: Client host rejected: Access denied'