Hi Mahesh1986,
Following are the possiblities that causes the issue.
1. Network connectivity needed and no firewall block the connection.
So disable the firewall and check.
Use PowerShell to test the connection.
Test-NetConnection ldap.domain.com -Port 636
2. Trust certificate is needed. If it's using a self-signed certificate, then it may not be trusted from the computer that you're downloading it from.
If it is not from trusted, then you will need to obtain the root certificate and install it as a Trusted Root Certificate on the machine where the code runs.
For creating certificate refer MSDN.
3. The fully-qualified domain name must match the SSL certificate exactly.
LDAP://ldap.domain.com:636
Reference:
https://stackoverflow.com/questions/54987776/ldap-connection-error-the-server-is-not-operational-when-connecting-to-port
4. The Microsoft SSL provider, selects the first valid certificate that it finds in the local computer store. If there are multiple valid certificates available in the local computer store, Schannel may not select the correct certificate.
5. If an existing LDAPS certificate is replaced with another certificate, either through a renewal process or because the issuing CA has changed, the server must be restarted for Schannel to use the new certificate.
