Well guys, thanks for your tips and pointers so far, trying to narrowing down the problem:
Zooming in on opendkim; the service is not running and cannot be started:
[root@server1 ~]# systemctl start opendkim
Job for opendkim.service failed because the control process exited with error code. See "systemctl status opendkim.service" and "journalctl -xe" for details.
[root@server1 ~]# systemctl status opendkim.service
● opendkim.service - DomainKeys Identified Mail (DKIM) Milter
Loaded: loaded (/usr/lib/systemd/system/opendkim.service; disabled; vendor preset: disabled)
Drop-In: /etc/systemd/system/opendkim.service.d
└─nethserver.conf
Active: failed (Result: exit-code) since Sun 2018-10-07 15:22:35 CEST; 9s ago
Docs: man:opendkim(8)
man:opendkim.conf(5)
man:opendkim-genkey(8)
man:opendkim-genzone(8)
man:opendkim-testadsp(8)
man:opendkim-testkey
http://www.opendkim.org/docs.html
Process: 25053 ExecStart=/usr/sbin/opendkim $OPTIONS (code=exited, status=78)
Oct 07 15:22:35 server1.pa3hfj.nl systemd[1]: Starting DomainKeys Identified Mail (DKIM) Milter...
Oct 07 15:22:35 server1.pa3hfj.nl opendkim[25053]: /etc/opendkim/keys/default.private: open(): No such file or directory
Oct 07 15:22:35 server1.pa3hfj.nl systemd[1]: opendkim.service: control process exited, code=exited status=78
Oct 07 15:22:35 server1.pa3hfj.nl systemd[1]: Failed to start DomainKeys Identified Mail (DKIM) Milter.
Oct 07 15:22:35 server1.pa3hfj.nl systemd[1]: Unit opendkim.service entered failed state.
Oct 07 15:22:35 server1.pa3hfj.nl systemd[1]: opendkim.service failed.
Then I decided to make an adjustment in one off the emaildomains by enabling the maildisclaimer and inserting a text. On activation the opendkim service came to live!
Oct 7 15:53:05 server1 httpd: [WARNING] NethServer\Module\Mail\Domain\Modify: file_get_contents(/var/lib/nethserver/mail-disclaimers/taalapart.nl.raw): failed to open stream: No such file or directory
Oct 7 15:53:05 server1 httpd: [WARNING] NethServer\Module\Mail\Domain\Modify: file_get_contents(/etc/opendkim/default.txt): failed to open stream: No such file or directory
Oct 7 15:53:05 server1 httpd: [WARNING] NethServer\Module\Mail\Domain\Modify: file_get_contents(/etc/opendkim/default.txt): failed to open stream: No such file or directory
Oct 7 15:53:18 server1 httpd: [WARNING] NethServer\Module\Mail\Domain\Modify: file_get_contents(/var/lib/nethserver/mail-disclaimers/taalapart.nl.raw): failed to open stream: No such file or directory
Oct 7 15:53:19 server1 /sbin/e-smith/db[30851]: /var/lib/nethserver/db/domains: OLD taalapart.nl=domain|AlwaysBccStatus|disabled|Description|Taal Apart|DisclaimerStatus|disabled|TransportType|LocalDelivery|UnknownRecipientsActionType|bounce
Oct 7 15:53:19 server1 /sbin/e-smith/db[30851]: /var/lib/nethserver/db/domains: NEW taalapart.nl=domain|AlwaysBccStatus|disabled|Description|Taal Apart|DisclaimerStatus|enabled|TransportType|LocalDelivery|UnknownRecipientsActionType|bounce
Oct 7 15:53:19 server1 /sbin/e-smith/db[30851]: /var/lib/nethserver/db/domains: OLD taalapart.nl=domain|AlwaysBccStatus|disabled|Description|Taal Apart|DisclaimerStatus|enabled|TransportType|LocalDelivery|UnknownRecipientsActionType|bounce
Oct 7 15:53:19 server1 /sbin/e-smith/db[30851]: /var/lib/nethserver/db/domains: NEW taalapart.nl=domain|AlwaysBccStatus|disabled|Description|Taal Apart|DisclaimerStatus|enabled|OpenDkimStatus|disabled|TransportType|LocalDelivery|UnknownRecipientsActionType|bounce
Oct 7 15:53:19 server1 httpd: [WARNING] NethServer\Module\Mail\Domain\Modify: file_get_contents(/etc/opendkim/default.txt): failed to open stream: No such file or directory
Oct 7 15:53:19 server1 httpd: [WARNING] NethServer\Module\Mail\Domain\Modify: file_get_contents(/etc/opendkim/default.txt): failed to open stream: No such file or directory
Oct 7 15:53:20 server1 esmith::event[30865]: Event: domain-modify taalapart.nl
Oct 7 15:53:20 server1 esmith::event[30865]: expanding /var/lib/nethserver/sieve-scripts/unknown.sieve
Oct 7 15:53:20 server1 dbus[894]: [system] Activating via systemd: service name='org.freedesktop.timedate1' unit='dbus-org.freedesktop.timedate1.service'
Oct 7 15:53:20 server1 systemd: Starting Time & Date Service...
Oct 7 15:53:20 server1 dbus[894]: [system] Successfully activated service 'org.freedesktop.timedate1'
Oct 7 15:53:20 server1 systemd: Started Time & Date Service.
Oct 7 15:53:20 server1 esmith::event[30865]: expanding /etc/opendkim.conf
Oct 7 15:53:20 server1 esmith::event[30865]: expanding /etc/opendkim/SigningTable
Oct 7 15:53:20 server1 esmith::event[30865]: expanding /etc/opendkim/KeyTable
Oct 7 15:53:20 server1 esmith::event[30865]: expanding /etc/opendkim/TrustedHosts
Oct 7 15:53:20 server1 esmith::event[30865]: expanding /etc/postfix/virtual
Oct 7 15:53:20 server1 esmith::event[30865]: expanding /etc/postfix/master.cf
Oct 7 15:53:21 server1 esmith::event[30865]: expanding /etc/postfix/internal_access
Oct 7 15:53:21 server1 esmith::event[30865]: expanding /etc/postfix/main.cf
Oct 7 15:53:21 server1 esmith::event[30865]: expanding /etc/postfix/transport
Oct 7 15:53:21 server1 esmith::event[30865]: expanding /etc/postfix/disclaimer
Oct 7 15:53:21 server1 esmith::event[30865]: expanding /etc/postfix/recipient_bcc
Oct 7 15:53:21 server1 esmith::event[30865]: expanding /etc/dovecot/dovecot.conf
Oct 7 15:53:21 server1 esmith::event[30865]: Action: /etc/e-smith/events/actions/generic_template_expand SUCCESS [1.346568]
Oct 7 15:53:23 server1 esmith::event[30865]: Action: /etc/e-smith/events/domain-modify/S30nethserver-mail-postmap-update SUCCESS [1.805474]
Oct 7 15:53:23 server1 systemd: Reloading.
Oct 7 15:53:25 server1 systemd: Starting DomainKeys Identified Mail (DKIM) Milter...
Oct 7 15:53:25 server1 systemd: Started DomainKeys Identified Mail (DKIM) Milter.
Oct 7 15:53:25 server1 esmith::event[30865]: [INFO] opendkim has been started
Oct 7 15:53:25 server1 systemd: Reloading.
Oct 7 15:53:25 server1 esmith::event[30865]: [INFO] service postfix restart
Oct 7 15:53:25 server1 systemd: Stopping Postfix Mail Transport Agent...
Oct 7 15:53:25 server1 systemd: Starting Postfix Mail Transport Agent...
Oct 7 15:53:26 server1 systemd: Started Postfix Mail Transport Agent.
Oct 7 15:53:26 server1 systemd: Reloading.
Oct 7 15:53:26 server1 esmith::event[30865]: [INFO] service dovecot reload
Oct 7 15:53:26 server1 systemd: Reloaded Dovecot IMAP/POP3 email server.
Oct 7 15:53:26 server1 esmith::event[30865]: Action: /etc/e-smith/events/actions/adjust-services SUCCESS [3.470779]
Oct 7 15:53:26 server1 esmith::event[30865]: Event: domain-modify SUCCESS
So one step further, i feel were getting close! 
But, still no joy at sending mail:
Oct 7 16:15:27 server1 postfix/pickup[31029]: D545912801E8: uid=0 from=<no-reply@server1.pa3hfj.nl>
Oct 7 16:15:27 server1 postfix/cleanup[32408]: warning: connect to Milter service unix:/var/run/opendkim/milter: Permission denied
Oct 7 16:15:27 server1 postfix/cleanup[32408]: D545912801E8: milter-reject: CONNECT from localhost[127.0.0.1]: 4.7.1 Service unavailable - try again later; from=<no-reply@server1.pa3hfj.nl>
The same permission denied messages on /var/run/opendkim/milter are in the log. The difference is that milter exists now, with opendkim:mail permissions:
[root@server1 opendkim]# ll
total 4
srwxrwx--- 1 opendkim mail 0 Oct 7 15:53 milter
-rw-r--r-- 1 opendkim mail 6 Oct 7 15:53 opendkim.pid
Is it worth a try to do a complete reboot to get things on its place?