On using Spotlight search, user still gets to see the deleted mail
Apple pushed the iPhone OS 3.1 SDK for the iPhone platform developers just a fortnight after releasing the 3.0 Software update in June. Of course there were several bugs in iPhone OS 3.0 software update and the most recent one is that it doesn't delete mails from the Mail app entirely. On using Spotlight search, user still gets to see the deleted mail.
CultofMac website reader Matt Janssen found a bug in iPhone 3.0 software that enabled retrieving of deleted email from the mail app. Evidently, Mail app in iPhone and iPod Touch devices doesn't delete a mail entirely since the mail can be easily found using Spotlight search in iPhone OS 3.0 software update running devices.
Here's a video demonstration in which Janssen shows how the deleted mail shows up in the built-in search tool.
Gizmodo's John Herrman tried all the necessary steps to remove mail from his iPod and even restarted it. But the deleted mail still showed up in search and thereby concluded that there exists a security flaw. So, at the moment, there's no way to delete any mail completely from iPhone OS 3.0 update running devices. However, you can always take the pains of restoring the iPhone OS 3.0 update again to the device and re-sync everything again.
This might be one of the many undiscovered security flaws that might be fixed when Apple releases iPhone OS 3.1 software update sometime in September.