Hello Bernard Polman,
Thanks for reaching out. Filter clause is applied as '?$filter= instead of '?filter' as used in your endpoint URL, hence the filter clause is ignored in your case. You can find more information on the filter clause here.
Please note filter clause may not be supported for all properties. In my tests filtering did not work with the 'emailaddress' property however works with the id property for example:
You can provide your feedback and suggestions at the Graph feedback forum.
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Thanks.