This is what ILSpy says about MailForWindowsPhone.dll:
// MailForWindowsPhone, Version=3.0.14228.1017, Culture=neutral, PublicKeyToken=6dc438ab78a525b3
// Architecture: Any CPU (64-bit preferred)
// Runtime: .NET 4.0
....
[assembly: TargetFramework("WindowsPhone,Version=v8.0", FrameworkDisplayName = "Windows Phone 8.0")]
So I'm not really sure what you might be doing wrong, or why VS is complaining.