Follow these steps :

Step.1- Include  MessageUI.framework in your project. 

Step-2  Import  MessageUI/MessageUI.h in your header file like that in my case

#import <MessageUI/MessageUI.h>
#import <UIKit/UIKit.h>
#import <QuartzCore/QuartzCore.h>  
@interface SendEmail : UIViewController<MFMailComposeViewControllerDelegate>
@property (retain, nonatomic) NSString *emailSubject;
@end

Setp-3 Suppose you want to send SMS on button  touch(click) then write the following code in your implementation (Sendemail.m) file. 

  MFMailComposeViewController  *mailcontroller =[[MFMailComposeViewController alloc] init];
  mailcontroller.mailComposeDelegate =self; 
// CHECK DEVICE CAN SEND EMAIL
 if([MFMailComposeViewController canSendMail]){ 
// Email subject. 
      mailcontroller.subject=@“this is email subject. ”; 
//  Email body 
   mailcontroller.body=[NSString stringWithFormat:@“ This is the testing email. “];  
// Email address you want to send . 
        mailcontroller.recipients = [NSArray arrayWithObjects:@”example@.com”,nil];        
// check the condition in case of model view controller is not dismissed then dismiss it. 
                if (![[self presentedViewController] isBeingPresented]) {
                    [self dismissViewControllerAnimated:YES completion:nil];                 
                }
// show Email view  UI.
[self presentViewController:mailcontroller animated:YES completion:nil];
            }  
// delegate method of  MFMailComposeViewController .  
-(void)mailComposeController:(MFMailComposeViewController *)controller
         didFinishWithResult:(MFMailComposeResult)result error:(NSError *)error{   
      [controller dismissViewControllerAnimated:YES completion:nil];  
}