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Certificate from GoDaddy is in place, I have removed esmtp inspection from our Cisco ASA, and hybrid mail is flowing from Office 365 to our on prem Exchange 2013 server. When trying to send mail to a mailbox migrated to Office365 I am receiving the dreaded '451 5.7.3 STARTTLS is required to send mail".

Since our MX is pointing to a 3rd party mail filter I changed the on prem connector created by the hybrid config wizard to relay mail through 'ourdomain-com.mail.protection.outlook.com'

I am also able to telnet directly from our Exchange server to ourdomain-com.mail.protection.outlook.com 25. An ehlo shows the STARTTLS verb IS available.

Kind of hitting a wall here. I would greatly appreciate any insights from the community!

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  • I forgot to add that we are also not blacklisted (checked via mxtoolbox). Thanks!
    – Christopher Dallas
    Jun 8, 2016 at 19:59

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In our network we have a router out in front of our ASA.

I had overlooked CBAC smtp inspection. I removed this statement on the router, re-ran the HCW, modified the On Prem connector to 365 to point to domain-com.mail.protection.outlook.com, and now Hybrid is working as intended.

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