Dovecot 2.4 - Issue with mail_location after Upgrade

[Andreas Günther]

Hello everyone,

I'm contacting you again regarding upgrade challenges with Dovecot from 
2.3.19.1 to 2.4.1-4 as part of a Debian system upgrade from 12 to 13.

Since I had over time put pretty much everything into `/etc/dovecot/
dovecot.conf`, I now want to cleanly integrate it into the files under conf.d/ 
in order to also get the quota issue (my post this week) working properly.  
Basically, everything worked perfectly in production with version 2.3.19.1 and 
the settings in the main Dovecot file. This should continue to be the case.

Here you can see the current /etc/dovecot/dovecot.conf file:
```Bash
dovecot_config_version = 2.4.1
!include conf.d/*.conf
verbose_proctitle = yes
namespace {
  list = yes
  location = maildir:%%h/:INDEXPVT=~/Shared/%%u
  prefix = Shared/%%u/
  separator = /
  subscriptions = no
  type = shared
}
remote 127.0.0.1 {
  disable_plaintext_auth = no
}
ssl_prefer_server_ciphers = no
```

And this is what the `10-mail.conf` looks like:
```bash
mail_driver = maildir
mail_home = /var/vmail/%d/%n
mail_location = maildir:~/Maildir
mail_path = %{home}/mail
mail_inbox_path = /var/mail/%{user}
namespace inbox {
  type = private
  prefix = 
  inbox = yes
  
  separator = /
}
mail_privileged_group = mail
mail_plugins = quota quota_clone acl fts fts_solr
mail_plugins {
  plugin = yes
}
protocol !indexer-worker {
}
```

Dovecot cannot be started now because "mail_location" is causing problems at 
this point. As shown here:
```bash
doveconf -n
doveconf: Fatal: Error in configuration file /etc/dovecot/conf.d/10-mail.conf 
line 39: mail_location: Unknown setting: mail_location
```

If I comment out this line and instead insert this line back into /etc/
dovecot/dovecot.conf, then this error is not caused.
  
Does the `mail_location` variable no longer exist in the new version, or how 
can this be resolved?   
I'm seeing similar behavior with several other variables; as soon as they're 
not in the main file but in their respective subfiles, error messages are 
thrown.

Can you suggest a solution?

Thank you very much,  
  
Andreas