July 1 Announcement of Exchange Server SE Launches the Subscription Era for Exchange Server

Right on schedule, July 1 saw the Exchange engineering team celebrating Microsoft’s new fiscal year by announcing the general availability of Exchange Server Subscription Edition (SE). I’m always suspicious about announcements just made at the end or start of a fiscal year because updates can be timed to satisfy artificial deadlines set by executives (to justify their bonuses). I don’t think that applies in this case because Exchange Server SE is a lightly rebranded version of Exchange 2019. At least, that’s what you might conclude by reading the slim list of changes (like a version number update).

Mentioning the Release to Manufacturing (RTM) build brought back memories of waiting for physical media containing a new release of Exchange Server, going right back to 1994 and the initial builds of “Touchdown” made available to customers. It was a charming look back into the past. Now we simply head online to grab the latest bits (Figure 1) and read the deployment instructions.

Moving to Evergreen Support

The big change is the move away from Exchange development based on three-year cycles (extended to six for Exchange 2019) to “evergreen” development. In some respects, the quarterly cumulative updates for Exchange Server showed the way forward in terms of keeping software refreshed. However, Exchange Server still followed the traditional support model based on versions whereas Exchange Server SE remains supported if customers keep the software refreshed with updates released by Microsoft.

Nine months ago, Microsoft flagged the end of support for Exchange 2016 and Exchange 2019 on October 14, 2025. After this date, Microsoft will no longer provide technical support for problems (aka bugs). The writing is on the wall: to continue in a supported state, customers must adopt Exchange Server SE or move to Exchange Online. Obviously, we’re now deep into the prime vacation period and thoughts might be more focused on suntan lotion than server upgrades, but this is an issue that cannot be overlooked, especially in hybrid environments where Microsoft requires on-premises servers that host connectors to Exchange Online to remain supported.

Email Bombs Away

If you don’t read the Microsoft Defender for Office 365 blog, you might have missed the update about protection against “email bombs.” Essentially, an email bomb is a form of attack against a mailbox where a large volume of messages (the bomb) hit a mailbox. The messages often originate from legitimate sources such as newsletters, but the target user never signed up receive the messages. Given the large quotas assigned to Exchange Online mailboxes, it’s unlikely that an email bomb will cause the mailbox to exceed quota, but the arrival of large numbers of unwanted and unexpected messages is certainly a distraction. And when someone’s distracted, they might make bad decisions, such as accepting help from an attacker who poses as a support representative.

In any case, an update to Microsoft Defender for Office 365 can monitor for the characteristics of email bombs, such as a sudden significant spike in the number of messages received by a mailbox. The spike is detected by comparison against the historical pattern of email traffic observed for the mailbox together with spam signals. When an attack is detected, Defender redirects the problem messages into the mailbox’s Junk Email folder. Microsoft says that they have blocked between 20K and 30K email bombs daily since the initial deployment of the technology in early May 2025.

Microsoft Defender for Office 365 includes a lot of useful protection against different types of email threat. For more details, including licensing, see the service description.

Ongoing Updates

Email security is never a static topic. Whether you’re upgrading on-premises servers to the latest version of Exchange Server to harden servers against external attack or deploying new software to detect and deflect attacks, this is an area we need continuous focus on. The joys of staying up-to-date!

So much change, all the time. It’s a challenge to stay abreast of all the updates Microsoft makes across the Microsoft 365 ecosystem. Subscribe to the Office 365 for IT Pros eBook to receive monthly insights into what happens, why it happens, and what new features and capabilities mean for your tenant.