help with warning

I keep getting this warning...

any idea what it means?

Warning: mail(): Failed to connect to mailserver at "localhost" port 25, verify your "SMTP" and "smtp_port" setting in php.ini or use ini_set() in c:\domains\serenitydesigns.ca\wwwroot\feedback.php on line 355
An error has occured, please report this to the website administrator.

thanks a bunch!!!