Your iOS device's Mail app includes a unified mailbox feature that lets you see the content of all of your email account inboxes in a single mailbox. It's accessed by tapping on the All Inboxes cell from the Mail app on your device.
In some cases, you may want to see all of the emails from two or more of your email accounts, but not all of them, and this is where a new jailbreak tweak called AllInboxesFilter is going to come in handy for heavy email users with jailbroken devices.