E-mail top level domain suffixes not incorporated in core e-mail validation function valid_email_address in Drupal

Using the Drupal core function valid_email_address($mail) doesn't cover all possible e-mail address syntax errors.

This function doesn't take the Top Level Domain in count. The explanation from the Drupal core developers is that they want to make e-mail addresses on a domain without a TLD, like root@localhost, valid.

So beware of this! This is spread all over the Drupal system. Even while registering on a Drupal website, you don't have to specify the TLD. So I think a lot of people will make the typo and register with an address like johndoe@hotmailcom and they will never receive their confirmation e-mail.