The most common reasons for not being able to send emails through Outlook are wrong Outlook settings or blocked port 25 by the Internet Service Provider.

Here are a few checks you can make to ensure the problem is not on your end:

Wrong Outlook settings

Some time it seems that Outlook is not configured correctly for that reason outgoing mail can not be delivered. You should configure Outlook setup properly with the below mention informations.

Use the correct Mail Client settings:

          Incoming mail server (POP3/IMAP server): mail.yourdomain.com
          Outgoing mail server (SMTP server): mail.yourdomain.com
          Username: the full e-mail address like [email protected]
          Password : Put correct password or asked Administrator
          SMTP authentication must be enabled for the login to be successful.
          IMPORTANT: Please do not use secure authentication to your mail server.

The standard port used for SMTP connections is 25. One of the most common reasons for not being able to send messages is that your Internet service provider blocks this port. This is why using an alternative SMTP port is highly recommended.

Check whether your ISP blocks port 25:

You should click on the Start menu button, there is a tab called "Run". In the text field of this window you need to type "cmd" and a window will appear with "command prompt". Just type the following command in the command prompt window:

for SMTP:      telnet yuordomain.com 25

Results from the 25 port check

If the test is successful, you should receive a message similar to this one:

220-serv01.sitegroundXXX.com ESMTP Exim 4.63 #1 Wed, 18 Apr 2008 07:17:30 -0500
220-We do not authorize the use of this system to transport unsolicited,
220 and/or bulk e-mail.

If you get a message saying that the connection is refused we recommend that you use the alternative port for your SMTP connection or informe your ISP.