If you are using Apache/PHP/MySQL, this is an error message which you can often see and can occur for a couple of reason.
1. The error is generated by the Apache server. The SSI module in the Apache is activated and you have into your page something that resemble to a directive for the SSI. In this situation the Apache will try to interpret that part of your page but fails with the reported error message.
Read more at http://httpd.apache.org/docs/2.0/howto/ssi.html#config
2. In my personal experience, you are trying to send an email using your own scripting language (like php mail function), and there is a default timeout set in php.ini. Usually, when you have a huge amount of emails to send (for example, sending emails to all forum members), you may get this message since the timeout has been reached and no more emails will be sent. It may also be due to the fact that your webhost has set a limit on how many emails you are able to send in an hour.
In most cases, you should seek help from your webhost as they know more about what to do. Let them know what you were trying to do when you got this error message. I’ve had luck with my webhost and they were able to increase my initially 50 emails per hour to 500 emails per hour.