This short guide shows you how to solve the error "Get-Mailbox : Active Directory server is not available" on a Microsoft exchange server. You may get the following error while executing the Get-Mailbox function on a Microsoft Exchange Server 2016.
The full error message is
Get-Mailbox : Active Directory server is not available. Error Message: Active directory response: The LDAP server returned an unknown error.
Properly cross-check that the Active Directory server is up and running in your environment. Try to ping an Active Directory server from Microsoft Exchange Server.