in an mu-plugin:
add_filter( 'enable_post_by_email_configuration', '__return_true', 100 );
That will enable it for you fine.
I don't think you are offering up enough practical information for someone to act on to solve this problem.
If you are wanting someone to create their own plugin to turn this on, you need to offer up more details on how to accomplish this task. Correct me if I am wrong, but I think this is more usable ...
@Phillippp -- What Peter wants you to do is write your own plugin so you can turn on this functionality without hacking the core files. Here is how you do it:
First, create a plain text file called activate-pve.php (you can really call it anything, it just needs the .php at the end). Now, within this file add this:
* @package Activate_PVE
* @version 1.0
Plugin Name: Activate Post via Email
Plugin URI: http://wordpress.org
Description: This activates the post via email functionality in a multisite network.
Author URI: http://wordpress.org
add_filter( 'enable_post_by_email_configuration', '__return_true' );
Save this file and upload this into your plugins directory (root domain directory > wp-content > plugins). Once it is in there, you should be able to go to your WordPress backend and activate it in your plugin list.
After that, you should be able to go to your Settings tab, then Writing, and the Post via Email option should show up for you. I hope this helps.