Filter hook for custom validation

Hi, I want to build a 2-step contact form, where the second step asks the user to repeat his/her email address entered in the first step. Before submitting the form, I want to check that the email addresses entered in step 1 and step 2 are the same.

I guess a filter hook into form validation, that allows me to add the logic for performing the address comparision, would do. Since I cannot find it in the documentation – can you tell me what the name of the filter is? Or is there another way to do what I want?