Our mail servers have very high usage and sends thousands of emails every hour. For this reason, our mailserver IP address could be black listed by some providers and emails that you will send may not be delivered.
To fix this problem, you should use your Internet Service Provider's SMTP server to send email. Contact your ISP and they will let you know which SMTP server to use for email sending.
Alternatively, you can signup with google apps (free), and move your email service to google (they are very reliable and it is recommended to use them for your email sending/receiving needs).
You can start with google apps from here: http://www.google.com/apps/intl/en/group/index.html
After you will sign up with google apps and confirm your domain, you will need to change MX record of your domain to ASPMX.L.GOOGLE.COM. This can be done from the "Edit MX Record" section on your control panel. In 24 hours after you have changed your MX record, you can start enjoying quality and free email service provided by google.