Each SOAP message is contained in an "envelope" that includes a header and a body. The header may include the message ID and date the message was sent, while the body contains the actual message. Because SOAP messages all use the same format, they are compatible with many different operating systems and protocols. For example, a user can send a SOAP message from a Windows XP machine to a Unix-based Web server without worrying about the message being altered. The Unix machine can then redirect the message to the appropriate location or open the file using a program on the system. While most SOAP messages are sent over the Web via HTTP, they can also be sent via e-mail, using SMTP.