This article describes how to test why you cannot send out emails, but you can receive them without any problems.

Steps to take:

1.) If you have a Microsoft Windows machine, then click on Start -> Run
2.) Type in there cmd or command, if cmd doesn't work, and hit enter.
3.) A black window, in form of a small box will open up, in there type:
     telnet yourdomainname 25 and hit enter
     (make sure yourdomainname is in form of domainname.com, for example if your domain name is ezpublishing, then you would type telnet ezpublishing.com 25)
4.) Once you hit enter, the screen should clear and it should show on the top:
     220 someserver.com ESMTP and a blinking cursor on the bottom of that.
     If this shows up as described, then your smtp is working. If not proceed to step 5.

5.) If you don't the message described in step 4, but instead you get an error, then the
     problem is on your ISP's side. Please contact them and ask them why they're
     blocking outgoing port 25 to other servers.