To reduce the huge volumes of spam created from insecure web sites, our servers don't allow mail to be sent via PHP mail(). Instead use either sendmail or SMTP. It's easy to set up in Joomla or WordPress.
Login to the admin area of your site, and click System > Global Configuration.
In the Mail Settings section, select Mailer as "Sendmail", fill in the From Email as any actual mailbox you have set up in control panel and make sure the sendmail path is /usr/sbin/sendmail -t -i.
Sendmail will only work if the mailbox you specify in "From Email" exists.
For WordPress you first install an SMTP plugin such as SMTP Mailer
Once installed, enter the settings as follows replacing yourdomain.com with your actual domain and the email address and password for an actual example that you have set up in your control panel.
You can then use the Test Email tab to send yourself a test message.
If your web site is custom made or another type of system than WordPress or Joomla, speak to your web master about how to verify that it can send email securely.