So, our email hasn’t been going into a black hole https://michaelellerbeck.com/2013/04/09/mimecast-getting-ready-to-start-sending-outbound-using-their-smart-host/

I took the time to spin up a 2008 R2 Domain Controller and use that for LDAP with mimecast. Once we fixed the typo in the firewall rules than they were able to contact the DC just fine. So far setup has been very smooth with mimecast