The download link from the e-mail takes you to "Purchased Products" page that shows you support contract info and lists the products you purchased.
For each product, two files are available:
The Windows installer (for example RebexSecureMail-2015R3.1-Full.exe) - this is what developers using Windows should download and install. The installer installs the DLLs, samples and documentation, and (optionally) registers DLLs with Visual Studio, making it possible to easily pick the DLLs in the "Add reference…" dialog.
A ZIP file (for example RebexSecureMail-2015R3.1-Full.zip) - this is mostly intended for developers using Linux or OS X. The ZIP file contains the DLLs and samples as well, but can’t perform any other automated action.
Both of these packages contain the DLLs. In the ZIP file, they are in the "bin" folder. The installer installs the same folder in "c:\Program Files (x86)\Rebex Components 2015 R3.1" (or wherever you choose to install to).
The "bin" folder contains three subfolders (and a platforms.txt files that describes their contents). Each subfolders contains DLLs for a different platform. The available folders/platforms are:
net-2.0 - binaries for .NET 2.0, .NET 3.0, .NET 3.5 and Mono
net-4.0 - binaries for .NET 4.0, .NET 4.5 and Mono
netcf-2.0 - binaries for .NET Compact Framework 2.0
netcf-3.5 - binaries for .NET Compact Framework 3.5
netcf-3.9 - binaries for .NET Compact Framework 3.9
xamarin.android - binaries for Xamarin.Android
xamarin.ios - binaries for Xamarin.iOS
Most developers target Microsoft Windows and the DLLs in “net-4.0” folder are what they need to reference from their projects. These are for .NET Framework 4.0/4.5. These DLLs are usually installed into "c:\Program Files (x86)\Rebex Components 2015 R3.1\bin\net-4.0"