Aaaaghh!
The dreaded email blacklisting!
For some strange reason, there's a server in between your computer and that of the intended recipient which thinks that you're a spammer. Occasionally that can occur when a poorly-configured spam filter identifies
all mail from a particular email provider as potential spam. (e.g. the system correctly identifies spam originating from an address ending in @yahoo.co.uk and wrongly blocks mail from all other addresses ending in @yahoo.co.uk). I once had to try half a dozen different email services until I could find one which wasn't blocked by the service used by an US-based ABer. (If you're having a similar problem all you can do is to try a different email provider's service).
More commonly though, spam filters block mail from particular IP addresses (or from a particular block of IP addresses). So the first thing to try is simply to change your IP address. Assuming that your ISP uses 'dynamic' addresses (which they almost certainly do), that's dead easy:
Start by finding out what your current IP address is by clicking here:
http://whatismyipaddress.com/
Then disconnect your router from the phone line (or cable), wait for a minute or so and reconnect it. Click on the same link to confirm that your IP address has changed, then try sending the email again.
The next thing to check is whether SMTP authentication is enabled in Windows Live Mail. (Some spam-filtering systems look for it). To do so, right-click on your account name and select 'Properties'. Click the 'Servers' tab and put a tick next to 'My server requires authentication'. Click 'Settings' and select 'Use same settings as my incoming mail server'. Click 'OK'.