Используйте - (NSString *)stringByAddingPercentEscapesUsingEncoding:(NSStringEncoding)encoding
в строке NSString, содержащей HTML, например:
NSString *html = @"<html><body><b>test</b></body></html>";
NSString *email = [NSString stringWithFormat:@"mailto:?subject=SnapShot&body=%@",[html stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding]];
[[UIApplication sharedApplication] openURL:[NSURL URLWithString:email]];