What email service do you have?
Depending on the account, this may or may not be possible.
Go to Settings > Mail, Contacts, Calendars and choose your account.
The next part is tricky. You need to find the Advanced section, and it isn't always in the same place. Try tapping on your Account name, you may have to do it again on the next screen. When you find it, tap on Advanced.
With luck you'll see a Mailbox Behaviors section, and a Deleted Mailbox option. Tap on it.
There should be two lists, which may or may not have anything in them. One will be On My iPad, the other will be On The Server. Setting the Trash to On My iPad is the more reliable method, but will mean your deleted files will be removed from the server. I believe this is also the default.
Try choosing a Trash folder on the Server, and see if that works. That way (if you set your cell phone to the same trash folder) you can retrieve mistaken deletes on either device.
Good Luck,
