Sender Policy Framework (SPF) is a simple email-validation system designed to detect email spoofing by providing a mechanism to allow receiving mail exchangers to check that incoming mail from a domain comes from a host authorized by that domain's administrator
Ask your DNS administrator of your "yourdomain.nl" to add an extra record. A SPF is an record in extention "txt", the text should be between quotes.
Example SPF record
"v=spf1 a mx include:carerix.net ?all"
Explaination SPF record:
v=spf1 - This txt record will be used for SPF sends mail from this domain
mx - Every server that is mentioned as mx server for "yourdomain.nl" will have permission to send mail
include:carerix.net - Every server of the domain "carerix.net" will be able to send mail
?all - When a server does not comply with the above then it will be answered "neutral"
http://mxtoolbox.com/spf.aspx om eenvoudig uw SPF record te zien
https://www.spfwizard.net/ (extra information for creating SPF records)
Using Digital Signing?
We suggest to add an extra SPF record in such case giving rights to that server too to send out mail for your domain. SPF should be added with: "include:intraofficesigning.com"