How to Make mail application for ios how and what i need to know

#import <MessageUI/MessageUI.h>
#import <MessageUI/MFMailComposeViewController.h>
        MFMailComposeViewController* controller = [[MFMailComposeViewController alloc] init];
	if (controller==nil) {
		return;
	}
	controller.mailComposeDelegate = self;
	[controller setSubject:@"Your subject"];
	[controller setMessageBody:@"Your message body" isHTML:NO];

I hope it's usefull

See You can access built in application what iDevice has Provided, for that you can go through this link. I think it will help full to you.

http://www.chupamobile.com/tutorial/details/65/Accessing+Built-In+Applications/[^]

Or you can create your custom mail application & create your Custom UI for Users.