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Enterprise Software Thread, 451 - 4.4.0 STARTTLS is required in Technical; We run our own internal Exchange 2013 server. Recently our feeder school have moved to Office 365 and are receiving ...
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    451 - 4.4.0 STARTTLS is required

    We run our own internal Exchange 2013 server.

    Recently our feeder school have moved to Office 365 and are receiving a bounce-back message.

    Remote Server at emea01-internal.map.protection.outlook.com (10.174.64.25) returned '450 4.7.0 Proxy session setup failed on Frontend with '451 4.4.0 Primary target IP address responded with: "451 5.7.3 STARTTLS is required to send mail." Attempted failover to alternate host, but that did not succeed. Either there are no alternate hosts, or delivery failed to all alternate hosts.

    Has anyone else seen this?

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    Don't quote me on this, but it sounds like the Office 365 servers want incoming connections to be protected with SSL. This will mean that your Exchange server is going to need a valid public SSL certificate to authenticate itself to the Office 365 servers. Having not used Office 365 extensively, I can't say if you can turn off requiring STARTTLS.

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