Not a Rant About Microsoft’s Plan to Stop Old Exchange Servers Sending Email to Exchange Online

Clarifying Why Some Unsupported Exchange Servers Need an Upgrade

Yesterday, I was walking the dog and listening to the March 29 edition of the Windows Weekly podcast featuring Paul Thurrott and Richard Campbell. Typically, I listen to pass time without needing to engage my brain too highly, but then Richard mentioned that I could deliver a good “half-hour of rant” about Microsoft’s grand plan to force customers to upgrade unsupported Exchange servers.

I can’t deny that I have been known to rant in the past, maybe even when hosted by Richard on his RunAs Radio talk show, but that’s when I am pointed to a microphone and Richard goads me into action. In this case, I think it might be a reflection that people are struggling to understand what’s going on. Certainly, a fair degree of miscomprehension is apparent in some of the comments posted to Microsoft’s announcement. Let me try to summarize what’s happening without ranting even a little bit.

What Microsoft is Doing with Unsupported Exchange Servers

First, Microsoft is not targeting every on-premises Exchange server. You can absolutely continue to run on-premises Exchange if that’s the best option for your organization. However, if you have a hybrid organization, the rules of the game are changing to force you to use supported software to send email from the on-premises side.

Initially, Microsoft is targeting on-premises Exchange servers with two characteristics:

  • The servers run unsupported software. Any Exchange 2007 or Exchange 2010 server is now unsupported. Exchange 2013 servers become unsupported on April 11, 2023.
  • The servers send email to Exchange Online over an inbound connector of the on-premises type. In other words, the problem servers act as the routing point of contact with Exchange Online – Microsoft knows about the servers because they’re part of a hybrid organization connected to Exchange Online. These servers are also connected to the internet (otherwise they can’t route email to Exchange Online) and are therefore vulnerable to attack, and because they route messages direct to Exchange Online, they can be the vector used by attackers to inject malware into Exchange Online.

Servers that do not handle the transmission of email to Exchange Online via an inbound connector are unaffected. Anything that happens inside the privacy of an on-premises organization is up its administrators. For now, you could even connect in some Exchange 5.5 servers running a Wolfpack cluster if you wanted – if the server handling email to Exchange Online runs supported software.

Microsoft says that “The enforcement system will eventually apply to all versions of Exchange Server and all email coming into Exchange Online.” This seems a little harsh but it is intended to make sure that email flowing into Exchange Online is safe. The way things seem likely to pan out is that Microsoft will gradually bring Exchange 2010, Exchange 2013, Exchange 2016, and Exchange 2019 into the program. After they’ve made sure that only Exchange servers running supported software can communicate with Exchange Online, Microsoft will extend the requirement to all Exchange servers that communicate with Exchange Online using any means. In other words, even servers that communicate with Exchange Online via an intermediary are subject to throttling and then blocking.

The final stage is to protect Exchange Online against any server that sends email to Exchange Online over SMTP. I’m not quite sure how Microsoft plans to validate that remote SMTP servers are up to scratch, but that’s where they seem to be heading. This part of the plan is likely more of a long-term strategy than a well-defined plan. Practical issues such as identifying what is and is not a supported version of any particular SMTP server that communicates with Exchange Online need to be resolved.

The end game is to ensure that Exchange Online is not exposed to malware or other issues that come in from external servers (outside Exchange Online). In many respects, this is no different to what happens today when Exchange Online Protection blocks spam and malware. Judgement is passed at a server level rather than individual messages.

Initial Focus on Exchange 2007

The initial focus is on Exchange 2007 servers (Figure 1). As you might expect, this is a very small subset of servers in hybrid organizations. I’ve heard that there might be a couple of thousand servers in this category worldwide. Exchange 2007 reached end of life six years ago (April 2017). It has not received any support or security patches since.

These servers are vulnerable to a wide range of known threats. They should not be in active use. The potential exists that an attacker could compromise these servers and use this route to attempt to penetrate Exchange Online. This is the crux of the matter: Exchange Online will not accept email from organizations that transmit email to Exchange Online using obsolete and vulnerable Exchange servers.

Exchange 2007 run in a world where less external threat existed

Unsupported exchange servers
Figure 1: Exchange 2007 run in a world where less external threat existed. Now it’s an unsupported Exchange Server

Blocking of Unsupported Exchange Servers Starts in July

Microsoft will use a three-phase report-throttle-block process to “encourage” customers to upgrade the problem servers. Details are in this article. Microsoft will start to throttle traffic from Exchange 2007 servers in June and move to block traffic from those servers in July. It is entirely the responsibility of tenant administrators to respond before a block descends on their on-premises email to Exchange Online. Three options are available:

  • Upgrade the problem server(s) to a supported version of Exchange Server (2016 or later, patched with the latest cumulative and security updates). This might involve replacement hardware. The load imposed by mail routing to Exchange Online is not likely to stress modern hardware, so a low-end server will suffice.
  • Move the on-premises side of the inbound connector to a server running a supported version of Exchange Server.
  • Direct email from Exchange on-premises to Exchange Online via a third-party mail gateway. (note: if the third-party gateway uses unsupported Exchange servers, its traffic is liable to be blocked).

In any of these cases, it makes absolutely no sense to keep vulnerable Exchange servers in production. It’s time to let Exchange 2007 die. Software designed twenty years ago simply cannot cope with the threat that exists today.

Microsoft is clear that Exchange 2007 is only the start. After they finish dealing with Exchange 2007, they will move on to Exchange 2010 and then Exchange 2013 servers that send email to Exchange Online over inbound connectors. It’s probable that the program will extend to Exchange 2016 and Exchange 2019 servers (that are not kept updated) as they age, and maybe even encompass third-party servers with known problem configurations.

The point is that the project is all about closing a potential attack vector into Microsoft 365. Just like stopping people using basic authentication to connect to Exchange Online (now almost done), this is the right thing to do.

Nothing to do with Consumer Email

Some reaction to the announcement focuses on spam generated from Microsoft cloud accounts. I believe this refers to consumer email accounts. At least one incident occurred where Exchange Online was hijacked and used for spam, but most spam does come from consumer accounts. Microsoft could tighten the use of consumer ( accounts for email, but that’s got nothing to do with the server initiative.

ISVs and Inbound Connectors

Speaking of inbound connectors, in February 2023 Microsoft disabled the ability of new Exchange Online tenants to activate inbound connectors of the on-premises type. This caused a bunch of problems for ISVs that depend on being able to route email for processing to a service that they run before sending messages back to Exchange Online for final delivery. The application of email signatures by a company like Code Two Software is a good example.

Microsoft has now issued guidance about how to handle the issue. Essentially, they have whitelisted some ISVs to reduce the friction caused by the restriction. In other cases, you’ll need to request activation through Microsoft support. According to the ISVs I have spoken with, the new scheme is acceptable. Let’s hope that this proves to be the case in practice.

Microsoft will hold an Ask Me Anything event on May 10 at 9AM PST on the topic of the Exchange Online transport enforcement system. For more details, check out this page. If you have any further questions, that’s the place to bring them.

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3 Replies to “Not a Rant About Microsoft’s Plan to Stop Old Exchange Servers Sending Email to Exchange Online”

  1. If Microsoft would have used “inbound connector” as many times as you in this article it sure would be less confusing 😊 I know they say transport based at some point, but for me it didn’t click. Also, when they finally say that it will target ALL emails but will start with 2007 version using inbound connector, doesn’t that imply any email in the future? Although there are probably no reliable ways to determine correct version for external email. But if admin didn’t spoof it somehow and it says Exchange 2007, then i guess it should be good to block.

    1. The scheme could extend to all email in the future. I think what Microsoft means is that “all email sent by on-premises Exchange servers to Exchange Online.” The question is whether they mean the servers that send the email to EXO (those that host connectors) or all Exchange servers in:

      – hybrid deployments or
      – any on-premises Exchange server

      I have asked for clarification.

    2. I asked for clarification and believe that Microsoft wants (over the long term) to move to a point where Exchange Online will only accept inbound email from compliant servers. Quite how long it will take to get to that point remains unknown. What’s sure is that Exchange on-premises servers that transmit email to Exchange Online will need to run supported versions, starting with Exchange 2007 and moving through the different versions.

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