目前，我的项目在 IOS 开发目标 3.2 上运行良好，但一旦我将开发目标 4.0 或更高版本。它给了我错误。以下是错误：
Ld /Users/taxsmart2/Library/Developer/Xcode/DerivedData/MessageUI-cmsggqfkxarvsnaizxfrkvsmggwj/Build/Products/Debug-iphonesimulator/MessageUI.app/MessageUI normal i386
cd /Users/taxsmart2/Desktop/ipad
setenv MACOSX_DEPLOYMENT_TARGET 10.6
setenv PATH "/Developer/Platforms/iPhoneSimulator.platform/Developer/usr/bin:/Developer/usr/bin:/usr/bin:/bin:/usr/sbin:/sbin"
/Developer/Platforms/iPhoneSimulator.platform/Developer/usr/bin/llvm-gcc-4.2 -arch i386 -isysroot /Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator4.3.sdk -L/Users/taxsmart2/Library/Developer/Xcode/DerivedData/MessageUI-cmsggqfkxarvsnaizxfrkvsmggwj/Build/Products/Debug-iphonesimulator -F/Users/taxsmart2/Library/Developer/Xcode/DerivedData/MessageUI-cmsggqfkxarvsnaizxfrkvsmggwj/Build/Products/Debug-iphonesimulator -F/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator3.1.3.sdk/System/Library/Frameworks -F/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator4.3.sdk/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks -F/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator4.3.sdk/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks -F/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator3.1.3.sdk/System/Library/Frameworks -F/Users/taxsmart2/Desktop/ipad/libxml -F/Users/taxsmart2/Desktop/ipad/ASIHttpRequest/Frameworks -filelist /Users/taxsmart2/Library/Developer/Xcode/DerivedData/MessageUI-cmsggqfkxarvsnaizxfrkvsmggwj/Build/Intermediates/MessageUI.build/Debug-iphonesimulator/MessageUI.build/Objects-normal/i386/MessageUI.LinkFileList -mmacosx-version-min=10.6 -Xlinker -objc_abi_version -Xlinker 2 -lxml2 -D__IPHONE_OS_VERSION_MIN_REQUIRED=40300 -framework UIKit -framework CoreGraphics -framework MessageUI -framework Foundation -framework CoreFoundation -framework MobileCoreServices -framework SystemConfiguration -lz.1.2.3 -framework CFNetwork -o /Users/taxsmart2/Library/Developer/Xcode/DerivedData/MessageUI-cmsggqfkxarvsnaizxfrkvsmggwj/Build/Products/Debug-iphonesimulator/MessageUI.app/MessageUI
ld: warning: directory not found for option '-F/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator3.1.3.sdk/System/Library/Frameworks'
ld: warning: directory not found for option '-F/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator3.1.3.sdk/System/Library/Frameworks'
ld: warning: directory not found for option '-F/Users/taxsmart2/Desktop/ipad/ASIHttpRequest/Frameworks'
Undefined symbols for architecture i386:
"_OBJC_CLASS_$_MessageWebLayer", referenced from:
objc-class-ref in MessageUIAppDelegate.o
ld: symbol(s) not found for architecture i386
collect2: ld returned 1 exit status
同样在上面的错误显示中与githubMessageWebLayer上的相同。那么我需要在构建设置或开发目标中进行哪些更改才能在 iOS 4.0 或更高版本上运行。我在构建设置中尝试了很多更改，但是 github 中的链接说 MessageWebLayer 在 4.0 或更高版本中不受支持。任何人都可以在这件事上帮助我吗？
如果此代码不可能，那么 iPhone 中的其他哪些类允许我在应用内检索电子邮件？