Relevant OTRS issues:
1100085

Works for me on KE14 (pykolab 0.7.28). What policy do you have in kolab.conf?

As for the resources, in the code I see this module support only three policies: ACT_MANUAL, ACT_ACCEPT and ACT_ACCEPT_AND_NOTIFY

Update: user invitation policy handling works, at least I can get invitation declined. It is declined either setting ALL_REJECT or ACT_REJECT.

According to Enabling SMTP Content Filters it should be ALL_MANUAL, not ACT_MANUAL (this one must be coming from Kolab2 probably). It doesn't mention anything about resources being handled differently, so it should work the same way as for users, so it is expected that all policies work for resources. I can't get resource reservation rejected based on invitation policy on 0.7.28 and earlier versions.

Yes, ACT_MANUAL is an alias for ALL_MANUAL in invitationpolicy module, but as I've said resources module supports only three policies. We should probably add support for ALL_* aliases, but adding support for all policies supported by invitationpolicy module would not be just a simple fix. That's why I'd like Architecture & Design to take a look at this.

In addition it is not clear what ALL_MANUAL policy means for resources. There is nobody behind the resource, so nobody can manually react to the reservation request. Even more, resource is calendar, so it has no mail type folder to deliver request.

The resources module is a different one from the invitation policy module. This ticket concerns the invitation policy module.

What's the problem with adding ALL_REJECT as a policy and handle it accordingly?

As I understand setting kolabinvitationpolisy: ALL_REJECT for resources does not work as for users, i.e. does not respond with Declined status.

I was referring to this code: https://git.kolab.org/diffusion/P/browse/master/wallace/module_resources.py;1c4d7bb4154143752af6e2542cdbfaf2f7372cb5$60

Again, the resources module is a different module from the invitation policy. I read this ticket as concerning the invitation policy. I'm not sure how the resources module got in to the mix.

As discussed with Jeroen REJECT does not make much sense for resources. So, I don't see anything in the code that would need to be fixed now. Consider updating documentation to be more precise.

Can you please elaborated why REJECT does not make sense?
There is a use when reservation requests to meeting room should be allowed to particular users only. VIP car is allowed to certain users to be reserved is yet another case I can think of.
Please share all the logic behind resource reservation then.