If emails are not being received by the intended recipient there are a number of items to consider.  Usually, if there is a problem with sending the email out of your mail server, you would receive quite a detailed error message which will include, somewhere in the text, the reason that the emails are not going out of your server.

Check the following:

1. Are the settings for your outgoing mail server correct.
    Use http://mxtoolbox.com/diagnostic.asp to check that your SMTP (outgoing) mail server details are correct.