If you get this error when sending mail from your website, it usually means one of these:

• SMTP data is incorrect (double-check username and password, make sure that you specified correct host and port).
• Firewall on your hosting server blocks outgoing connections from specified SMTP host and/or port. Contact your hosting support and describe this problem to fix the issue.
• Specified SMTP host (in case if it is a domain — not IP) is not resolved on your hosting server or resolved to a wrong IP address. Contact your hosting support and describe this problem to fix the issue.

For more details on this issue please refer to this page.