Bounce bei "Host not found"

[Helmut Schneider]

Hallo,

ich habe einen Postfix Server mit 3 Instancen, default, in und out. in
nimmt Mails für einen Exchange entgegen. Der Postfix spricht mit dem
Exchange via OpenVPN.

Wenn der Tunnel nicht aktiv ist (passiert ab und an) gehen Mails
verloren:

Apr  1 05:12:44 h2786452 postfix-in/smtp[2747]: 6912C3E044A:
to=<info at me.local>, relay=none, delay=24, delays=0.02/0.02/24/0,
dsn=5.4.4, status=bounced (Host or domain name not found. Name service
error for name=exchange01.me.local type=AAAA: Host not found)

Wie kann ich Postfix sagen, dass er bei der internen Zustellung gnädig
sein soll und die Mails deferren und nicht bouncen soll, (vergebliche)
Relayversuche aber gerne ablehnen kann?

strict_rfc821_envelopes = yes
smtpd_delay_reject = yes
smtpd_helo_required = yes
smtpd_client_restrictions =
    permit_mynetworks
    permit_sasl_authenticated
    warn_if_reject check_client_access
hash:/etc/postfix-in/client_access
    warn_if_reject reject_unknown_client_hostname
    warn_if_reject reject_unknown_reverse_client_hostname
    warn_if_reject reject_rbl_client ix.dnsbl.manitu.net
    warn_if_reject reject_rbl_client zen.spamhaus.org
smtpd_helo_restrictions =
    permit_mynetworks
    permit_sasl_authenticated
    warn_if_reject check_helo_access hash:/etc/postfix-in/helo_access
    warn_if_reject reject_non_fqdn_helo_hostname
    warn_if_reject reject_invalid_helo_hostname
    warn_if_reject reject_unknown_helo_hostname
smtpd_sender_restrictions =
    permit_mynetworks
    permit_sasl_authenticated
    warn_if_reject check_sender_access
hash:/etc/postfix-in/sender_access
    warn_if_reject reject_non_fqdn_sender
    warn_if_reject reject_unknown_sender_domain
smtpd_recipient_restrictions =
    permit_mynetworks
    permit_sasl_authenticated
    reject_unlisted_recipient
    reject_unauth_destination
smtpd_data_restrictions =
    permit_mynetworks
    permit_sasl_authenticated
    reject_unauth_pipelining

unknown_address_reject_code = 554
unknown_client_reject_code = 554
unknown_hostname_reject_code = 554

Danke!