Values not updated in the GUI frontend of a JEE application using the Apache Wicket framework

I am using the Apache Wicket 6.0 framework as the GUI framework in my web application. I have the following problem: after changing the data, the value displayed in the text field does not change according to the data that has been entered respectively calculated after entering it and pressing a button to perform a calculation. Unfortunately, I have to refresh the site. Why does this behavior occur?

Validate an Email address

It's a common task to integrate email addresses to a web application. But how do we check them? How to make sure it's a valid Email Address? On which level?
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