outlook

Ok – hands up – I’m a complete novice and dont even do Computing stuff but I’m at a loss. Can somebody out there please help me. I’m just an ordinary user and cant use my outlook. I can receive but not send. I get 550 Administrative prohibition message when it bouces back. I’m not sending to a new / old / full account. What I have been told so far is that Messenger has “filled up” and “taken over”. Messenger now un-installed, it STILL DOESNT WORK…any ideas?