Keep getting "invalid email" message on any Civicrm/Joomla operation, such as register new individual or org address, or anything requiring email
This is fixed in 5.21. If you want to apply the patch now it's a small patch:
https://github.com/civicrm/civicrm-packages/pull/271/files

In fact for a quick temporary fix all you need to do is change that line 58 from

$part = idn_to_ascii($part, 0, INTL_IDNA_VARIANT_UTS46);

to

$part = idn_to_ascii($part);

More info: https://lab.civicrm.org/dev/core/issues/1469