When sending a message with senders other than aliasses used by you, you receive a non-delivery-report with this message:
„Sender address rejected: not owned by user“
To prevent address forgery and misuse, you can only send e-mails with senders that are registered for your account as mail address or alias (Settings => mailbox.org => e-mail aliases).
If you try to use senders which we cannot assign to your account or which are not at mailbox.org at all, the e-mails will be rejected with the error message:
553 5.7.1 <firstname.lastname@example.org>: Sender address rejected:not owned by user email@example.com;
Normally you should only use sender addresses, which you have also registered as mail address/alias, so that you can also receive replies or bounce messages for these addresses.
However, in the corporate environment it can sometimes make sense to use many different sender addresses, for example, to assign processes or tickets to a single customer or because automated booking systems send many e-mails.
These addresses should also be registered individually as aliases. However, if a catchall is configured for the account for this domain (@example.com), logically any user name of this domain can be sent - and it‘s then also ensured that replies can actually be delivered to these addresses.