There are two reasons that this might be happening:

1. The authentication of the server failed. If you are using Microsoft Outlook for your email, you will need to authenticate the server.  If you are using webmail directly, your email is authenticated automatically when you log in to your account.

2. Your ISP is blocking Port 25.  If your ISP (internet service provider) is blocking port 25, this will prevent you from using the standard SMTP port to send emails. Some ISPs do this so that you are forced to use their email servers when sending email in an attempt to prevent spam. Some ISPs that have been known to block port 25 are Comcast, Earthlink, and AT&T. A quick test and possible way around the block is to try changing the SMTP port setting in your email software. Change the port to use port 587, and then try to send emails again.

 
Testing the Port 25 Connection:

1. If you have a Microsoft Windows operating system, then click on Start -> Run.

2. Type in 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 the box, type “telnet example.com 25” and hit enter (replace example.com with your domain name and extension).

4. Once you hit enter, the screen should clear and it should show on the top “220 someserver.com ESMTP” and below you should see a blinking cursor. If this shows up as described, then your smtp is working. If not proceed to step 5.

5. Contact your ISP (internet service provider) and ask them why they're blocking outgoing port 25 to other servers.