After setting up a port forward for 443 to a internal server, now all hosts behind the NAT cannot access anything outside the network on port 443.

I setup the port forward with:
/ip firewall nat add chain=dstnat dst-port=443 action=dst-nat protocol=tcp to-address=10.9.8.91 to-port=443

And yes.. I know I misspelled outbound in the subject. *sigh*
On a brighter note.. the inbound port forward is working fine.

Add in-interface=WAN or dst-address=WAN-IP to your nat configuration, and everything should work fine.