a CGI/Perl script on a Web server) sending a request to an Internet Mail Server, to check if the email address exists and if a connection would be accepted by the Mail Server.
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Everyone must have filled an online form at some stage, a form usually asks for information related to name, phone no, address, credit-card no etc.
Incase you didn't provide information in the format specified by the form field or leave it empty, the message will appear and form cannot be submitted until you get it right.
For the majority of your users, Java Script form validation will save a lot of time up front, but double-checking the data on the server remains necessary, in case the user has turned Java Script off.