Outlook might not work for various reasons. First thing you should try is restart outlook and/or reboot windows. This fixes about 10% of the issues.

If this doesn't work then it could be a setup issue. Follow these steps in order to check things :

Click on Tools > Accounts > Mail

... then click properties on your favorite email.

Make sure your incoming and outgoing server match your domain name (without the "www." or "mail." prefix), check that your account name is your full email address and not just part of it and retype the password to make sure you got it right.
Do NOT use secure password authentication. Also your outgoing server does NOT require authentication (only incoming does) so you should uncheck that box.

On the advanced tab check that the numbers are as in the screenshot (incoming port might be 110 in case you use POP3)

If you got those settings right things should work smooth. If they don't then you could try too google the exact error code written in the outlook error.

One common issue is that some ISP-s block outgoing port 25 trying to limit the spam. You can test this by hitting Start > Run and type "cmd". A command shell will show up. Type there :

telnet yourdomain.com 25

You should get a welcome message like :
220 yourdomain.com ESMTP Sendmail 8.12.10/8.12.10; Fri, 10 Feb 2006 19:21:20 -0500

If you don't then your ISP is blocking port 25 (or maybe you have a firewall on your computer or router)
Talk to your ISP on how you can fix this issue.

If this still doesn't work then please contact support by submiting a ticket containing complete information about your setup (email, account name, password, protocol, incoming/outgoing server)