"Reading rights" in the native iOS email app

I'm very new to this iOS developing scene so please people, give me some credit, thank you!!
I'm thinking of developing an App and I need to read the incoming emails (after the approval and consent of the user!!).
So my question is if it is possible to read the message header and body if the user allows this on his iPhone? Again, I'm talking about the native iOS email app...
We also need to consider the other iOS email apps like Gmail, Yahoo and Hotmail...

Thank you very much to help me clarify this question!

You are not allowed to access other applications on the iOS system, that would include the Mail app.
this is strictly enforced by the operating system, and you own apps have to work within a sandbox, which means it can only access files and resources within it's own application bundle, this is the same with OSX apps as well, although you can use Applescript on OSX for accessing the Mac Mail app.

To do want your asking as regards the other email services, you would have to create your own email client application, that is a big project if your a beginner, and you would have to read up on the various SDK's from the email providers like Google, Yahoo, ect. Because each of these providers has there own developer communities and SDK's.

Sorry I could not help you more.

Regards Mark

Thank you very much Mark FX, very helpful answer!