Microsoft defines DOS as : "An attempt by a malicious (or unwitting) user, process, or system to prevent legitimate users from accessing a resource (usually a network service) by exploiting a weakness or design limitation in an information system. Examples of DoS attacks include flooding network connections, filling disk storage, disabling ports, or removing power." The message u got is not an error message. It is a warning message from Sajha. That message is prompted in Sajha if you: 1. Disrupt its server by sending more requests than it can possibly handle, thereby preventing access to its service, intentionally or unintentionally. 2. Use brute force attack continuously against the server. 3. Consume computational resources of the server, such as bandwidth, disk space, or CPU time. Sajha also does report your IP to your ISP in case of continuous attempts. Just in case, if due to some error (quite unlikely though) this message is generated, Sajha gives you the option of reporting it to admin@sajha.com.