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"