After upgrading to 5.20.0 - unable to edit or enter contact email addresses- "Email is not valid."
Thanks to the comments above, I have edited the packages/HTML/QuickForm/Rule/Email.php file to remove INTL_IDNA_VARIANT_UTS46. Forms with emails on them now work again.
Change: 
$part = idn_to_ascii($part, 0, INTL_IDNA_VARIANT_UTS46); 
to 
$part = idn_to_ascii($part);

This is how it was in the CiviCRM 5.19.2.