Reject recipient in postfix mail relay

The default value not altered.
smtpd_recipient_restrictions = permit_mynetworks, reject_unauth_destination

You are not sending from $mynetworks.
You are not sending to mydestination = $myhostname, localhost.$mydomain, localhost, $mydomain.

Perhaps you want:

smtpd_recipient_restrictions = permit_mynetworks permit_sasl_authenticated reject_unauth_destination
smtpd_tls_cert_file = /etc/ssl/postfix/cert.crt
smtpd_tls_key_file = /etc/ssl/postfix/key.key
smtpd_tls_CAfile = /etc/ssl/postfix/chain.crt
smtpd_tls_loglevel = 1
smtpd_tls_security_level = may
smtpd_tls_auth_only = yes

Modify cert values to be appropriate.

Setting up sasl is too variable to describe here, but http://www.postfix.org/SASL_README.html#server_dovecot should be a good start.

Your request is bit complicated because this matter. Postfix, by design, will execute something based on positive result from maps. For example in your transport_maps, it will send email to specific host if the recipient has positive result from maps lookup. But, you want to execute something - redirect to another mailbox - if there is negative result from your relay_recipient_maps lookup. With your maps type is hash, I think there are no other way unless involving sql/tcp map for implementing negative result logic in the map.

For sql solution, here is the idea:

In this example I'll use sqlite. Suppose that you have table with one column named relay_recipient. Now the query

select ifnull(select relay_recipient from mytable WHERE relay_recipient = %s, 'mailboxspam@example.com')

will return mailboxspam@example.com whenever the recipient doesn't exist in relay_recipient_maps. Place those maps on virtual_alias parameter. Then whenever the email sent to unknown user, then it will be aliased to mailboxspam@example.com