This one is tricky. You can try to use two short text questions Q1 and Q2. At first
use regular expressions to check if the user entered a valid email adress.
Then
set a condition so that each following question should only be shown if the anser to Q2 matches the answer of Q1. This is tricky but according to the manual it can work:
"Alternatively, you can fill this box with a @SGQA@ tag pointing at a previous question: then then condition will be evaluated against and older answer (for instance: display this question if the answer to the previous "Date-2 question" is greater than the answer to the previous "Date-1 question"
".
Copy this condition to each following question (conditions can easily be copied).
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