I do not know full pipeline, so my guess is mail is “routed”:
provider.net --> outlook.com (redirect) --> your MC
best solution is
provider.net --> your MC
or at least
For solving this at your MC, at least temporary is log into recipient@example.nl MC UI (assuming this is your mailbox), go into antispam settings and whitelist sender@provider.net (and/or sender@outlook.com not sure now,…). This way you should bypass this false-positive (for you). Just for this combination of recipient and sender. It’s most surgical solution that does not affect overall settings.
You may consider (bellow i do not recommend those,..)
That said you can change FREEMAIL_POLICY_FAILURE score, but this allow others bad actors easily bypass your antispam (not just from outlook.com, not just from providet.net, not just for recipient@example.nl)
Also you can add outlook.com as forwarding host in your MC but this allow (possibly) others bad actors from outlook.com bypass your antispam
[unknown] still very strange the result is only outlook.com gets rejected, other emails don’t.
You did not show any of those other senders, my assumption is that they are not classified as free mail, so they are not under this rule (FREEMAIL_POLICY_FAILURE). Or maybe they know how to send e-mail properly, starting whit not breaking ARC and other antispam features,..