Autodiscover.xml failed (0x80004005) and BPOS

A customer contacted to report a problem with Outlook’s AutoDiscover feature. The customer was a former BPOS customer that was testing some mailboxes deployed onto a new on-premises environment. The testExchangeConnectivity validation tool was reporting consistent results, so it appeared to be client related, but given the general complexity of the environment (domains, networks, VPNs,…