Its actually a good application that offers a basic feature which should, by now, be present in iOS... but what Electric Pocket Limited fails to mention is that Mailtones requires you to forward your mail to their servers, which will then bump it back to you with the appropriate alert sound.
Thats not the end of the world, but could be seen by some as a security risk - so it would have been nice to know this up front.
NakedMeeple about MailTones - Pager Alerts for Email and Gmail