you are setting the from address to come from hidden_e_post element, it does not look like this will yield a valid email address:
<input name="hidden_e_post" type="hidden" value="<?php if(isset($_SESSION['kt_e-post'])){ echo "Sähköposti: " . $_SESSION['kt_e-post'] . "<br>" ;}?>">
the from email address must be a valid email address