Referencing this old post of mine for a how-to:
https://www-secure.symantec.com/connect/forums/adding-javascript-clear-intial-value-onclick
Here's the steps I'd use:
1. Make the Control ID of the email textbox "myTxtBox" (no dbl quotes) under Functionality / Behavior
2. Add a AttributeKeyValuePair custom event to the Custom Events in Functionality / Behavior
3. Make the event onblur, and add the event handler script as follows (per your requirement):
var myTextBox = document.getElementById("myTxtBox");
if(myTextBox.value.indexOf("something.net", myTextBox.value.length - 13) < 0)
{
window.alert("Email must end in 'something.net'");
}
This will popup a message yelling at the user if they enter something that doesn't end in "something.net" for an email.