hi,
I thought this was important, in order to have responsible comments.
I'm not sure is there can be a fail-safe-check for e-mail addresses.. even because some mail servers accept messages to non-existent addresses!
Community Builder implements a mail check function when registering, so I started digging from there :p
The standard Joomla! PHPmailer library has the needed functions for checking, and the JMail class returns a conveniently filled mail structure.. so..

You may try this 2step approach to get the e-mail address validated

1. extract
http://www.kotasbiketeam.org/tmp/email-check.zip to your JComments folder (\components\com_jcomments\)
2. edit \components\com_jcomments\jcomments.ajax.php ~line 150
/*
} else if (!empty($values['email']) && (!preg_match( _JC_REGEXP_EMAIL2, $values['email']))) {
JCommentsAJAX::showErrorMessage($response, 'email', JText::_('ERROR_INCORRECT_EMAIL'));
*/
} else if (!empty($values['email'])) {
if (! is_file(JCOMMENTS_BASE.DS . 'email-check.php')) {
if (! preg_match( _JC_REGEXP_EMAIL2, $values['email']))
JCommentsAJAX::showErrorMessage($response, 'email', JText::_('ERROR_INCORRECT_EMAIL'));
} else {
require_once (JCOMMENTS_BASE.DS . 'email-check.php');
if (emailCheck($values['email']) == 1) {
$noErrors = true;
} else {
JCommentsAJAX::showErrorMessage($response, 'email', JText::_('ERROR_INCORRECT_EMAIL'));
}
}
} else if ...
Let me know if it worked, you may use it as you wish

regards,
luis raposo