Normally, Email To SMS will send out everything in the body of your Email. That means if you have a signature with a large disclaimer text, it will try to send that out too. In most cases, that means that the text is too long to send out, resulting in an error. That's why we have the <end> tag.
The <end> tag determines where your message ends and the rest of the mail begins. Use it at the end of the message that you want to send out. The <end> tag is optional.