for instance, when the recipient’s address is full (a soft bounce: just wait and re-send) or worst, when it’s non-existent (a hard bounce: you need to remove the account from your list)

Errors-To is deprecated, so mail servers will typically ignore this header - most servers will bounce will to the 'envelope sender'. This is the email address that your mail client sends as part of the connection to the SMTP server (not necessarily the From address - though it typically is the same).