.NET doesn't search Rebex Components install folders when loading referenced DLL, which means that in order to deploy your application, you should either deploy the referenced DLLs such as Rebex.Mail.dll (and its dependencies) with your application's executable into the same directory, or install the shared DLLs into Global Assembly Cache (GAC). The first option seems to be used more often nowadays.
Check out the related discussion on Stack Overflow for additional information.
Having a reference with Copy Local=true makes Visual Studio copy the references DLL into our projects's output folder (such as Debug or Release), so it should actually end up in the same folder as myservice.exe. If it's not, something is wrong somewhere...