Validate an email while this in the input field - when do you start?

1. How do I validate an email input field?
2. When should input validation occur?

How do I validate an email input field?

The <input type="email"> defines a field for an e-mail address. The input value is automatically validated to ensure it is a properly formatted e-mail address. To define an e-mail field that allows multiple e-mail addresses, add the "multiple" attribute. Tip: Always add the <label> tag for best accessibility practices!

When should input validation occur?

Data validation should occur in two locations: The point where data is acted upon, for example validating input parameters to an SQL query. General validation at the point where data is submitted, for example in a web application some validation should occur on the client.