So now you have an NDR with the answer to the problem in, you can fix it! It takes 1 to 2 hours to get delisted from when Microsoft process your email – so they say it takes 48 hours end to end.
The NDR for the rejection will tell you to email Microsoft’s delisting service. If you change your connector (temporarily) to allow opportunistic TLS or no TLS at all then your emails will leave the queue – but will be rejected by the Microsoft servers. The reason why you are not getting STARTTLS offered is that your connecting IP address is on the Microsoft block list. If you are not configuring hybrid or EOP standalone and need to send an email to someone on Office 365 then this is not an issue, because Exchange Server does not require TLS for normal email communication and so the lack of a STARTTLS verb means your email is sent in clear text. Communication between your on-premises servers and Microsoft for hybrid or EOP configurations requires TLS and if you cannot start TLS then your email will queue. STARTTLS is the SMTP verb needed to begin a secure and encrypted session using TLS. If you turn on protocol logging you get this error in the log “Connector is configured to send mail only over TLS connections and remote doesn’t support TLS” and if you look at the SMTP protocol verbs that are recorded in the log you see that Microsoft’s servers do not offer STARTTLS as a verb. You set up an Exchange Online Hybrid or Exchange Online Protection (EOP) stand alone service and follow all the instructions for the creating of the connectors needed, only to find that your emails queue in your Exchange Server.
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