I have found this is a common issue. 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. 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.
STARTTLS is the SMTP verb needed to begin a secure and encrypted session using TLS. 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.
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.
The reason why you are not getting STARTTLS offered is that your connecting IP address is on the Microsoft block list. 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 NDR for the rejection will tell you to email Microsoft’s delisting service. 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.
Therefore my recommendation when setting up Exchange Online Hybrid or stand alone EOP is to send an email over plain text to EOP before you configure your service. If you are on the blocklist then you will get back the delisting email and you can process that whilst setting up the connectors to Office 365 and so by the time you are ready to test, you are off the blocklist!
To send a test email over Telnet
- Install the Telnet Client feature on your Exchange Server that will be your source server for hybrid or connectivity to EOP for outbound email
- Type the following. This will send an email to a fake address at Microsoft, but will hit the TLS error before the message is rejected
telnet microsoft-com.mail.protection.outlook.com 25
- You are now connected to Exchange Online Protection and you should get a 220 response
- Type the following to send the email by command line. No typo’s allowed in telnet, so type carefully. Replacing your email address where prompted so that you get the NDR back to you.
mail from: firstname.lastname@example.org
rcpt to: email@example.com
from: Your Name <firstname.lastname@example.org>
subject: testing to see if my IP is blocked
type something here, it does not matter what, this is the body of the message you are sending
- A few points about the above. It must finish with a . (full stop) on a line by itself followed by a carriage return. There must be a blank line between the subject line and the body. And finally, for each line of data you type, the Microsoft SMTP servers will return either a 250 or 354 response.