You can get a quick overview on this page about the mail system's current status.
With this setting, you can switch the mail server for your domain on and off. If you manage the mail system using an external server, please deactivate the mail server for your domain here. If you do that, the MX record in the zone file must point to your external mail server.
Note: You can completely deactivate local mail inboxes/reception, but ONLY if the MX record points to an external mail server. If it does not point to an external server, there is a good chance that you will lose emails.
This shows the current number of mailboxes and redirects for all of the accounts, as well as how much disk space they currently are using.