The answer is similar to the one in the comment. Edit file qa-include/qa-app-format.php and change:
if (isset($postuserid) && isset($usershtml[$postuserid])) {
$whohtml=$usershtml[$postuserid];
if ($microformats)
$whohtml='<span class="vcard author">'.$whohtml.'</span>';
$userinfo = qa_db_select_with_pending(qa_db_user_account_selectspec($postuserid, true));
$domain = array_pop(explode('@', $userinfo['email']));
$whohtml .= ' ' . $domain;
} else {
Note this will work only if external users is disabled.