This e-mail checking script is tought in any javascript teaching course or book. This is a very simple script based on string manipulation. If you see the source code of this page, you will find a function "checkmail()" in the <script> tag in head. This function needs many variables, but you do not need to worry about variable declaration.
A valid e-mail address must contain a commercial at sign(@) and a dot ".". The script checks just for these two parameters in the string entered in the textbox. If the dot comes before at, script gives an error message. If the dot comes just after at, script gives an error. If the dot comes after the e-mail server's name, script gives an alert showing that the e-mail address is a valid one.