Upgrading VS2010 to 2013 introduced compile error with SendGrid - visual-studio

Question

I installed the ultimate trial of Visual Studio 2013 yesterday to see if it had any negative impact on our current 2010 based projects. When trying to compile a project that previously had no problems and still compiles on my colleagues computers I encountered this error:
The type or namespace name 'SendGridMail' could not be found (are you missing a using directive or an assembly reference?)
The error is the same when run from Visual Studio 2010 or 2013
Looking at similar questions on here suggests that this happens when using the .net 4 Client Profile instead of standard .net 4, but this project has always used standard .net 4.
Another question was using the latest version of sendgrid (4.x) with .net 4 which I understand doesn't work, but we are using an older version (1.2.1).
Personally I have tried:
Removing and Readding the reference (originally it was referenced inside the project, I also tried referencing it externally)
Running Visual Studio with no addons
Recreating the project in another solution
The only luck I have had so far has been to use nuget to install the sendgrid package instead but this seems to add a lot of additional files and at the moment I'm more interested in why a previously working build has stopped working with no changes to the project other than installing another version of Visual Studio.
Edit:
This is the output when trying to build with the SendGridMail dll (succeeds since it's not used in code):
C:\Windows\Microsoft.NET\Framework\v4.0.30319\Microsoft.Common.targets(1605,5): warning MSB3268: The primary reference "SendGridMail, Version=1.2.1.0, Culture=neutral, processorArchitecture=MSIL" could not be resolved because it has an indirect dependency on the framework assembly "System.Net.Http, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" which could not be resolved in the currently targeted framework. ".NETFramework,Version=v4.0". To resolve this problem, either remove the reference "SendGridMail, Version=1.2.1.0, Culture=neutral, processorArchitecture=MSIL" or retarget your application to a framework version which contains "System.Net.Http, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a".
C:\Windows\Microsoft.NET\Framework\v4.0.30319\Microsoft.Common.targets(1605,5): warning MSB3268: The primary reference "SendGridMail, Version=1.2.1.0, Culture=neutral, processorArchitecture=MSIL" could not be resolved because it has an indirect dependency on the framework assembly "System.Threading.Tasks, Version=2.5.19.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" which could not be resolved in the currently targeted framework. ".NETFramework,Version=v4.0". To resolve this problem, either remove the reference "SendGridMail, Version=1.2.1.0, Culture=neutral, processorArchitecture=MSIL" or retarget your application to a framework version which contains "System.Threading.Tasks, Version=2.5.19.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a".
C:\Windows\Microsoft.NET\Framework\v4.0.30319\Microsoft.Common.targets(1605,5): warning MSB3268: The primary reference "SendGridMail, Version=1.2.1.0, Culture=neutral, processorArchitecture=MSIL" could not be resolved because it has an indirect dependency on the framework assembly "System.Runtime, Version=2.5.19.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" which could not be resolved in the currently targeted framework. ".NETFramework,Version=v4.0". To resolve this problem, either remove the reference "SendGridMail, Version=1.2.1.0, Culture=neutral, processorArchitecture=MSIL" or retarget your application to a framework version which contains "System.Runtime, Version=2.5.19.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a".

Answer 1

I was able to resolve this by removing the .NET 4.5 SDK (but not the framework itself). I assume it was looking at the latest framework for the dlls despite the project being set to .NET 4