Hi, How to add constraint to require user to fill a correct email ? Does a question type email exist ?
You can use a regular string question with a constraint containing a suitable regex() to check for a valid email address. There's more about writing regex's here: https://docs.getodk.org/form-regex/
What does/not actually constitute a valid email is actual rather complicated [for more than you probably ever wanted to know, see https://www.regular-expressions.info/email.html], but a basic one may suffice in most cases; eg
^[A-Z0-9+_.-]+@[A-Z0-9.-]+$