By default an Exchange Server 2007 Hub Transport server only allows secure authenticated connections, so if you don’t have an Edge server in front of the Hub Transport server, you need to allow anonymous connections directly to the server.
You cannot set this authentication setting via the GUI (only when creating new connectors!), so you’ll need to open the Exchange Management Shell and type:
set-ReceiveConnector “Default <Servername>” -permissiongroups:”ExchangeUsers,ExchangeServers,ExchangeLegacyServers,AnonymousUsers”
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PowerShell One-Liner examples:
Get-Mailbox < mailbox ID> | Set-Mailbox –EmailAddressPolicyEnabled:$false
Get-Mailbox –Server SRV1 | Set-Mailbox –EmailAddressPolicyEnabled:$false