Error 550 - Livemail in Windows 7

[#1]

I have been trying to send an e-mail to a cousin, whose address I have checked, rechecked and has been entered correctly.
When I hit send -  'error 550'  comes up every time, and Livemail freezes until I delete the message from my outbox. Livemail then operates normally.
Incidentally, my cousin is with ukonline.co.uk
This happens only with this one particular address and I get no problems at all with everyone else's  e-mail.
It's not a life threatening problem, just very annoying. I just wondered if anyone else has come across this?
As a newbie to this forum, apologies if this subject has already been covered in the past.
Thanks
Bob

[#2]

Are you sure your cousin still has a ukonline email address? I recall mention of Sky closing the brand down completely around 12 months ago.
Has your cousin sent an email to you in the last few months? If so what sender address does it show?
David