First you need to complete the setup process of your custom domain.
Go to Settings -> Global Settings -> Custom email domains.
Next to each custom email domain is a three dot symbol. When you click it, you can select 'Set a catch all mailbox'. Then you need to choose to which email address and, consequently, to which of your inboxes catch-all mails should go to. This is usually your main admin, but can be any user that you have added to your account.
Once you have set up your catch-all email address, all emails that are being sent to your own domain that do not match any existing email address will be delivered to this catch-all inbox.
Tip: If your domain got leaked to spam lists, we recommend that you set up a dedicated user within your Tutanota account for receiving catch-all emails, e.g. firstname.lastname@example.org.
A catch-all email address lets you receive all emails sent to your custom domain – even if the sender mistyped your email address.
In Tutanota you can set up a catch-all email address with any paid subscription.