When trying to add proxy/additional email addresses to the AD template using the provisioning manager and apply the changes, the provisioning manager does not save the changes.
As the AD endpoint has been acquired without exchange gateway server, this AD endpoint is marked with no exchange option.
This means eTADSexchange=0 into the endpoint object definition in the provisioning store.
Specific context where AD is used without exchange but Client needs additional smtp addresses (eTADSproxyAddresses) in AD because they are required so to sync their AD with Azure.
This is a workaround.
set eTADSexchange=1 (Exchange managable) into the "eTADSDirectoryName=xxx,eTNamespaceName=ActiveDirectory,dc=im,dc=etadb" path DN of the provisioning store DB.