Windows – Can’t install additional roles and features on Windows Server 2012 R2 Standard because of missing source files

I have a Virtual Machine from VirtualBox running Windows Server 2012 R2 Standard which is used for some development tasks. I wanted to install an SMTP server, so I can send emails from there, however, everytime I do so I get an error page which looks like this

I used this link https://docs.microsoft.com/en-gb/biztalk/install-and-config-guides/appendix-d-create-the-smtp-server#install-smtp-server to get information on how to install the STMP server.

After this failed the first time, I searched the internet a bit, and found a solution which said that if Windows Update is deactivated, then you have to manually provide the sources, however, I have Windows Update installed and it works, since it checked this morning for updates.

Anyhow, I still downloaded the .iso of Windows Server 2012 R2 from https://www.microsoft.com/de-de/evalcenter/evaluate-windows-server-2012-r2#evaluation_521 and tried two things

1. Add the .iso via VirtualBox (Devices -> Optical Drives -> Choose a disk file…) so it was in the path D:\
2. Extact the .iso in my machine to a folder so it was in the path C:\Downloads\Win10

I again went through the installer and clicked "Specify an alternate source path" on the Confirmation screen and tried both solutions

but it still didn't fulfill the installation and had the same error as in the image above, stating that the source files are missing.

But as you can see in the following screen, you can see that it should work

Update 1

So what I then tried is to do this via PowerShell "Install-WindowsFeature -Name SMTP-Server -Source D:\sources\sxs" but this fails with

Install-WindowsFeature : The request to add or remove features on the specified server failed.
Installation of one or more roles, role services, or features failed.
The source files could not be downloaded.
Use the "source" option to specify the location of the files that are required to restore the feature. For more
information on specifying a source location, see http://go.microsoft.com/fwlink/?LinkId=243077. Error: 0x800f0906
At line:1 char:1
+ Install-WindowsFeature -Name SMTP-Server -Source D:\sources\sxs\
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (@{Vhd=; Credent...Name=localhost}:PSObject) [Install-WindowsFeature],
    Exception
    + FullyQualifiedErrorId : DISMAPI_Error__Cbs_Download_Failure,Microsoft.Windows.ServerManager.Commands.AddWindowsFeatureCommand

which is strange, since you can see that I'm using a source parameter. When I add /source it tells me that this parameter is not supported

I even tried it with Dism but when I use Dism.exe /online /Get-Features there is no listing of an SMTP Server and when I use Dism.exe /online /enable-feature /featurename:SMTP-Server /Source:D\sources\sxs /all it tells me that there is no feature named SMTP-Server

Update 2

I also followed this tutorial in here https://docs.microsoft.com/en-gb/windows-hardware/manufacture/desktop/configure-a-windows-repair-source#configure-group-policy-for-feature-on-demand to set my repair sources to be from Windows Update instead of WSUS but till the same error. :/