550 5.1.1 <admin@example.com>: Recipient address rejected: example.com

I'm trying to setup 2 mail-servers inside my Local Area Network, sender.com(ubuntu/LAPTOP) and example.com (dsm/NAS). Both are linux systems and both use postfix, the following goes wrong when sending a message from user@sender.com to admin@example.com

==============================

telnet localhost 25 220 sender.com ESMTP Postfix (Ubuntu)

EHLO example.com 250-example.com 250-PIPELINING 250-SIZE 10240000 250-VRFY 250-ETRN 250-STARTTLS 250-ENHANCEDSTATUSCODES 250-8BITMIME 250 DSN

mail from: user@sender.com 250 2.1.0 OK

rcpt to: admin@example.com 550 5.1.1 admin@example.com: Recipient address rejected: example.com

==============================

Master zone: example.com example.com MX 1 smtp.example.com smtp.example.com A 192.168.1.2 example.com A 192.168.1.2 m.example.com A 192.168.1.2 example.com NS www.example.com www.example.com A 192.168.1.2

Master zone: sender.com smtp.sender.com A 192.168.1.123 sender.com MX 1 smtp.sender.com sender.com A 192.168.1.123 www.sender.com A 192.168.1.123 sender.com NS www.sender.com

==============================

alias_database = hash:/var/packages/MailServer/target/etc/aliases alias_maps = hash:/var/packages/MailServer/target/etc/aliases broken_sasl_auth_clients = yes command_directory = /var/packages/MailServer/target/sbin config_directory = /var/packages/MailServer/target/etc cyrus_sasl_config_path = /var/packages/MailServer/target/etc daemon_directory = /var/packages/MailServer/target/libexec data_directory = /var/lib/postfix disable_vrfy_command = yes home_mailbox = .Maildir/ inet_protocols = ipv4, ipv6 mail_owner = postfix mailbox_command = /var/packages/MailServer/target/libexec/dovecot/dovecot-lda -f "$SENDER" -a "$RECIPIENT" mailbox_size_limit = 0 mailq_path = /var/packages/MailServer/target/bin/mailq message_size_limit = 10485760 mydestination = $myhostname, localhost.$mydomain, localhost myhostname = example.com newaliases_path = /var/packages/MailServer/target/bin/newaliases queue_directory = /var/spool/postfix recipient_bcc_maps = hash:/var/packages/MailServer/target/etc/bcc/recipient_bcc sender_bcc_maps = hash:/var/packages/MailServer/target/etc/bcc/sender_bcc sendmail_path = /var/packages/MailServer/target/sbin/sendmail setgid_group = maildrop smtpd_client_restrictions = check_client_access hash:/var/packages/MailServer/target/etc/access/client_access, permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination smtpd_recipient_restrictions = check_client_access hash:/var/packages/MailServer/target/etc/access/client_access, check_sender_access hash:/var/packages/MailServer/target/etc/access/sender_access, check_recipient_access hash:/var/packages/MailServer/target/etc/access/recipient_access, permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination smtpd_sasl_auth_enable = no smtpd_sasl_authenticated_header = yes smtpd_sender_restrictions = reject_unknown_sender_domain smtpd_tls_cert_file = /usr/syno/etc/ssl/ssl.crt/server.crt smtpd_tls_key_file = /usr/syno/etc/ssl/ssl.key/server.key smtpd_tls_security_level = may unknown_local_recipient_reject_code = 550