feat: Restore recommondation to avoid system mailbox

Usage of the system mailbox was discouraged before and
adds some issues when migrating mail setups.

Therefore make it more clear that this is provided
as a neat, quick feature, but not recommended for
regular e-mail communcation. Use a normal mailbox instead.

Author: pixelbrackets

source/mail-mailboxes.rst

@@ -123,9 +123,12 @@ System mailbox
 With each Uberspace you will get a mailaddress in the form ``[email protected]``. By default this mailaddress forwards
 to the external mailaddress you have given with your registration and we recommend to keep it that way.
 
-If you want to use the mailaddress as a mailbox, you need to remove the file ``~/.qmail`` and the Maildir will be
-created under ``~/Maildir`` with the first incoming mail. But as this is no ordinary mailbox, it wont work with the
-:doc:`Spamfolder <mail-spam>` or :doc:`Sieve filter rules <mail-filters>` or plussed mailaddresses.
+If you want to access the system mailbox, you may remove the file ``~/.qmail`` and the Maildir will be
+created under ``~/Maildir`` with the first incoming mail.
+
+.. warning:: As this is no ordinary mailbox, it wont work with the :doc:`Spamfolder <mail-spam>` or :doc:`Sieve filter rules <mail-filters>` or plussed mailaddresses.
+
+The system mailbox is rather intended as a fallback, and we do not recommend using it for regular communication.
 
 .. warning::
   In the past we also promoted mailaddresses in the form ``[email protected]``. This may not be