daknetworks.com

Here is the best source for setup of LDAPS:

https://blogs.msdn.microsoft.com/microsoftrservertigerteam/2017/04/10/step-by-step-guide-to-setup-ldaps-on-windows-server/

For Mimecast, if you are using a self-signed certificate as the instructions above provide, set the Encryption Mode to: Relaxed