The full error with callstack is:
2015-12-18 04:59:47.8391 Px2704 Tx32 A40 C209 M2501 Error Failed to cache source SMTP to Db | System.Exception: Failed to retrieve RootDSE at 'my_domain' under 'DMM_service_account' isGlobalCatalogReady property | at Quest.MigrationManager.Exchange.Basis.LdapHelper.GetDirectoryEntryRootDseInfoEntryPropertyIsGlobalCatalogReady(String domain, String username, String password)
at Quest.MigrationManager.Exchange.Basis.LdapHelper.get_domain()
at Quest.MigrationManager.Exchange.Basis.LdapHelper.GetByObjectGUID(Guid objectGUID)
at Quest.MigrationManager.Exchange.Basis.LdapHelper.GetMailboxLDAPInfo(Guid objectGuid)
at Quest.MigrationManager.Exchange.Agent.Jobs.Realm.get_MailboxInfo()
at Quest.MigrationManager.Exchange.Agent.Jobs.Realm.get_PrimarySmtp()
at Quest.MigrationManager.Exchange.Agent.Jobs.DisBaseJob.CacheSourceSmtp()
[inner] System.Exception: Failed to retrieve RootDSE entry at 'my_domain' under 'DMM_service_account' | at Quest.MigrationManager.Exchange.Basis.LdapHelper.GetDirectoryEntryRootDse(String domain, String username, String password)
at Quest.MigrationManager.Exchange.Basis.LdapHelper.GetDirectoryEntryRootDseInfoEntryPropertyIsGlobalCatalogReady(String domain, String username, String password)
[inner] System.Runtime.InteropServices.COMException: The server is not operational.
| at System.DirectoryServices.DirectoryEntry.Bind(Boolean throwIfFail)
at System.DirectoryServices.DirectoryEntry.Bind()
at System.DirectoryServices.DirectoryEntry.get_Name()