Moved to Email Gateway for better support, hopefully
There are two options to solve this. The first is to add the Exchange certificate to the MEG. This is done under Email --> Certificate Management --> Certificates --> CA Certificates. Simply add the certificate or whatever CA chain may be used on Exchange.
The second method is to disable certificate verification for LDAP. This is the check box at the bottom of Email --> Group Management --> Directory Services. Unchecking the box will allow the the secure connection to work even if the certificate is not validated.
Thank you very much, Your answer makes me sense, I will try it