@Roland Häder

I'm not really that deep into Dovecot.

But in the the file /etc/dovecot/conf.d/10-mail.conf there is the entry

mail_location = mbox:~/mail:INBOX=/var/mail/%u

This specifies how Dovecot should access an existing directory structure.

In my case the line is
mail_location = maildir:/var/mail/vhosts/%d/%n

which seems to fit your use too.
%d is the domain and %n is the user.

When I install Postfix and Dovecot, I always follow this tutorial:

How to install postfix an dovecot and roundcube

I checked every log file that exists - for what reason ever, only the "Email administrators on new user registration" mail doesn´t come through.
Ill try to check that tommorrow on my dev-instance.

BTW: i got even step debbugin working wird Friendica and xdebug. Im totally lost in the code but sooner or later i´ll get into it.

@Hypolite Petovan @Shiri Bailem
I am still completely lost in the code, but was able to identify the problem.

/src/module/register.php

Line 403:
DI::notify()->createFromArray.

will never reached because:
Line 402:

foreach (User::getAdminListForEmailing(['uid', 'language', 'email']) as $admin)

returns an empty array. Of course, if the array is empty, no mail will be sent.

Unfortunately, I don't know why the array is empty, but with the following changes to the sendNotification function, the email arrived immediately:

Original code:

foreach (User::getAdminListForEmailing(['uid', 'language', 'email']) as $admin){
				DI::notify()->createFromArray([
				'type'                      => Model\Notification\Type::SYSTEM,
				'event'                     => $event,
				'uid'                       => $admin['uid'],
				'link'                      => DI::baseUrl() . '/moderation/users/',
				'source_name'               => $user['username'],
				'source_mail'               => $user['email'],
				'source_nick'               => $user['nickname'],
				'source_link'               => DI::baseUrl() . '/moderation/users/',
				'source_photo'              => User::getAvatarUrl($user, Proxy::SIZE_THUMB),
				'show_in_notification_page' => false

Modified code (getAdminEmailList instead of getAdminListForEmailing and uid was set manually):

foreach (User::getAdminEmailList(['uid', 'language', 'email']) as $admin) {
				DI::notify()->createFromArray([
				'type'                      => Model\Notification\Type::SYSTEM,
				'event'                     => $event,
				'uid'                       => '2',
				'link'                      => DI::baseUrl() . '/moderation/users/',
				'source_name'               => $user['username'],
				'source_mail'               => $user['email'],
				'source_nick'               => $user['nickname'],
				'source_link'               => DI::baseUrl() . '/moderation/users/',
				'source_photo'              => User::getAvatarUrl($user, Proxy::SIZE_THUMB),
				'show_in_notification_page' => false

@Hypolite Petovan @Shiri Bailem
Got it.
I checked my dev instance and my prod instance. on both instances the parent ID of the admin is NULL but not 0.
a simple update user set `parent-uid` = '0' where uid ='2'; did the trick. Notification Emails are now arriving without changes to the code.

User::getAdminListForEmailing should be updated to check

parent-uid = 0 OR parent-uid = NULL

Obviously there are instances running where the parent-id is NULL.

My developer instance was a total fresh install - fresh os, fresh friendica and the parent-uid was NULL.

Anyway, we have a solution :)