Explain why it is important to check the email address with JavaScript and again when it reaches the server.
It is important to check the email address with JavaScript first on the client-side because it is quicker than the server and you don't need to refresh any pages (assuming AJAX is not used). Additionally, it is important to validate the email address on the server-side afterwards since the user could modify the page and bypass the front-end validation. This adds an extra layer of protection to ensure that only validated data is processed.
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