The text that you see used on Joomla pages is all contained in the language file. You can find it at:
/administrator/language/english.php (in my case).
I added some extra text to the definition to try and help the blocked email problem:
DEFINE('_REG_COMPLETE_ACTIVATE', '<div class="componentheading">Registration Complete!</div><br />Your account has been created and activation link has been sent to the e-mail address you entered. Note that you must activate the account by clicking on the activation link when you get the e-mail before you can login.
<br /><br />If you use hotmail or aol, please check your spam filters for the confirmation email. Until you confirm you will not be able to log in!');
You can use this trick of hacking the language file to personalize your site and make it more unique.