Use Fake implementations at debug time

Bootstrap.cs

var builder = new ContainerBuilder();

builder.AddModule(new ModuleA());
builder.AddModule(new ModuleB());

builder.AddModule(new DebugFakesModule());

var container = builder.Build();

// Will be FakeEmailService if compiled with DEBUG
var emailService = builder.Resolve<IEmailService>();

DebugFakesModule.cs

    class DebugFakesModule : Module
    {
        protected override void Load(ContainerBuilder builder)
        {
#if DEBUG
            builder.RegisterAssemblyTypes(ThisAssembly)
                .Where(x => x.Name.StartsWith("Fake"))
                .AsImplementedInterfaces()
                .SingleInstance();
#endif
        }
    }

MailService.cs

// Only gets used when compiled as a DEBUG compiler switch

class FakeMailService : IEmailService
{
  // Fake impl
}

class EmailService : IEmailService
{
  // normal impl
}