How you will find what IP has outlook.com if you drop traffic to DNS server?
Well, there are two things: the client can get a DNS server (actually resolver) where it can lookup outlook.com, this can be the MikroTik router itself when it is configured to forward those DNS requests to next level resolvers (e.g. at the ISP, or 1.1.1.1 or 8.8.8.8 for example). No need to allow any Microsoft addresses for that.
However, some software thinks it is "smart" by sending some packets to specific external addresses (in this case some DNS server maintained by Microsoft) irrespective of the settings on the system itself (done via DHCP or static).
This is sometimes part of a scheme to detect if there is a "wifi logon portal" (e.g. "hotspot") in the way that needs to be shown to the user to enter their credentials before the connection to the internet server can be made.
Such mechanisms often allow extra firewall rules so they work correctly, in this case maybe allow DNS forward which would normally be unnecessary and maybe even unwanted.