I register the domainname dompelingict.nl and I set the settings mxrecords from the internethost to 192.168.1.5
That is not correct. 192.168.1.5 is not a valid internet address. That is not the IP address of your Internet connection.
How is your server connected to the Internet?
You should also remove mx1.hostnet.nl. from the MX records of your domain. mx1.hostnet.nl. is not configured to receive mail for your domain (which is the direct reason for the rejection you are reporting), and it also will not be able to filter spam as effectively as your SME server.
[charlieb@localhost ~]$ host dompelingict.nl
dompelingict.nl has address 192.168.1.5
dompelingict.nl mail is handled by 100 mx1.hostnet.nl.
dompelingict.nl mail is handled by 10 mail.dompelingict.nl.
[charlieb@localhost ~]$
Please read the User Manual at least twice. If you follow the setup steps documented there, you should be able to receive mail for your domain.