[ms office - 2010] Outlook - conflict with hotmail.co.uk addresses

[speckytecky]

Got a strange issue on my Outlook Account (OS Vista Business SP2 with MS Office 2010). Whenever I try sending messages to [email protected] (tried at least 3 known working addresses) the sent box tells me it went OK but the recipients never get the message through on their system. Any thoughts on what the issue might be - no problems with any other addresses just the hotmail.co.uk one?