local network - postfix - MUA - mailx

I am testing postfix on a local network with sasl and tls.
In the last step the mail server would send mail outside.
Actually I don’t want to send mail outside.
During my test with sasl, I succeed to send a mail outside but I was expecting that my mail was rejected.
Currently I have put a dummy address in the relayhost parameter

relayhost = []

I would like to know if there is any parameter that I can set to be sure that no mails can go out.
I shall remove this parameter ( or change the value ) when I will be ready to send mail outside.

Any help is welcome.

After googleing I have found this workaround.

Sasl configuration :

smtpd_sasl_auth_enable = yes
smtpd_sasl_security_options = noanonymous
smtpd_sasl_local_domain = $mydomain
# Authorize relay for authenticated users
smtpd_recipient_restrictions = permit_sasl_authenticated,  permit_mynetworks, reject_unauth_destination


smtpd_recipient_restrictions = permit_sasl_authenticated,  permit_mynetworks, reject_unauth_destination


smtpd_recipient_restrictions =
  check_recipient_access hash:/etc/postfix/recipient_access,  reject_unauth_destination

Solve my problem :
If /etc/postfix/recipient_access is empty then recipent address is rejected :

NOQUEUE: reject: RCPT from hostname.mydomain.com[]: 554 5.7.1 <someuser@anotherdomain.com>: Relay access denied; from=<user_install@mydomain.com> to=<someuser@anotherdomain.com> proto=ESMTP helo=<hostname.mydomain.com>

If /etc/postfix/recipient_access is fill with :

# for relaying domain
# user@domain.com OK

non_existent_user@somedomain.com      REJECT
good_existent_user@somedomain.com    OK
somedomain.com                                   REJECT

Then the mail is sent.

Nota : mydomain.com does not exists. It is local.
The mx record is set by the router using dnsmasq config.
I have found a site where you got an arbitrary email address wher to send mails.
Mails are destroyed 60 mn later.
This way I can test if outbound mails are delivred or not.

I want to be sure during my tests that no mail goes outside until I use a isp as relay.

Any comments are welcome.