How to Delete All Sent Items on iPhone
The email application on an iPhone automatically saves all received and sent items to its server. To recover space on your email server, you can delete all of the email messages located in your "Sent" folder. When deleting messages, be sure you wish to delete the specific item because it cannot be recovered once it is erased from the mail server.
Delete Sent Items using iPhone
Press the "Home" button on your iPhone and slide your finger across the screen to unlock it.
Tap the "Email" icon, then tap the tab in the upper-left corner to go to the main email menu.
Select the "Sent" tab, and tap "Edit" in the upper-right corner.
Tap the circles next to all of the messages you wish to remove. Hit "Delete" at the bottom of the screen.
Delete Sent Items using MobileMe
Open your Internet browser and click the URL field. Type "me.com." Press "Return."
Type your username and password. Select "Log In."
Click the "Mail" icon and hit "Sent." Select the first message at the top of the screen and hold down "Shift." Scroll to the bottom of the "Sent" folder and click the last message to highlight the entire folder. Hit the red "Delete" button at the top of the screen. Once you delete the "Sent" Items from your MobileMe account, your iPhone will sync with the email server and erase the items from the device as well.
Delete Sent Items using Gmail
Open your Internet browser and click the URL field. Type "gmail.com" and press "Enter" on your keyboard.
Type in your username and password. Click the "Sign In" button.
Click the "Sent Mail" tab in the left sidebar. Choose "All" next to "Select."
Click "Delete." Hit "OK" in the dialog box. After you delete the "Sent" Items from Gmail, your iPhone will sync with the email server and erase the items from the device as well.