Compare with null
Currently, a filter like "$filter=mail eq null" fails with the error message
"Unsupported or invalid query filter clause specified for property 'mail' of resource 'User'."
According to the OData spec, comparison with the literal "null" is allowed, and only "null" is equal to "null".
A simple use-case:
In our AD, there might exists users that have no email address registered to them. We would like to find these users, but that would mean we have to filter out people with "mail eq null", which we cannot. The current solution to this is to fetch every single user (currently roughly 13000 users), and filter them on our end to find the users without an email address.
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Victor Bakke commented
Clarification: this is all on the Microsoft Graph API.