Yes, Mail.dll supports almost all .NET framework versions (except .NET 1.1, and .NET client profile).
It can run on: .NET 2.0, 3.0, 3.5, 4.0, 4.5, 4.5.1 and 4.5.2 and Mono.
If you use nuget package appropriate version is referenced automatically.
If you're adding library reference manually, you need to choose appropriate version yourself: in the Redistributes folder there are 2 subfolders: Net20 and Net45:
- Use Net20 version for frameworks: 2.0, 3.0, 3.5, 4.0;
- Use Net45 version for frameworks: 4.5, 4.5.1, 4.5.2 and above.
There is no difference between those two assemblies, simply different .NET target is used during compilation.
Mail.dll always runs on the framework you use in your application or have installed on your machine. .NET 4.5.2 can use .NET 2.0 assemblies, and they run in .NET 4.5.2. There is no performance or security impact referencing assembly compiled using previous .NET version.