Reply-All Storm Suppression Only for Large Cloud Tenants

Publicity for Exchange for a Change

I’ve been impressed by the amount of coverage given by mainstream IT reporters to Thursday’s announcement by the Exchange development group that they are rolling out a feature to suppress Reply-All storms. Few of the recent announcements by the Exchange group have received such attention, possibly because Teams hoovers up so much press interest with a constant stream of new features.

In this case, the interest is probably because people remember the 1997 “bedlam DL3” email storm. It’s the stuff of email legend. But it happened so long ago (when we used Exchange 5.0 and 5.5 servers and X.400-based MTAs) that the only surprising thing about this announcement is why it has taken Microsoft twenty-three years to come up with a technique to suppress email storms. Maybe it’s because an email storm for servers powered by early Pentium CPUs was a more serious problem than it is for a cloud service. Hundreds of thousands of mailbox servers come in handy sometimes.

Technology Stops People Doing Silly Things

Even in the era of the cloud, the root cause of the problem (user stupidity) hasn’t gone away. People are still likely to do silly things with technology, which is why it’s best when technology improves to stop people doing things they shouldn’t. For instance, Exchange 2010 introduced Mail Tips to advise users of conditions like sending messages to large distributions. It’s also the reason why Data Loss Prevention policies exist to stop people sending credit card information in email or sharing documents packed full of sensitive information.

Users don’t need to reply-all to a conversation to say: “take me off this list.” Instead, they can use Outlook’s ignore conversation feature to stop seeing any further replies. Better still, administrators can apply moderation to large distribution groups so that only specific people can send messages to these lists. Some simple PowerShell code is enough to find large lists and apply moderation to them. For example:


Moderation means that every message posted to the list must be approved. That could be a bit extreme, so another approach is to restrict the people who can send to an approved set.

Transport Pipeline is Where Magic Happens

Given all that’s been done to help users do the right thing, it seems strange that Microsoft did not move sooner to close off reply-all storms. All messages processed by Exchange (on-premises or online) go through a transport pipeline. Operations like address resolution and message bifurcation occur in the pipeline, as do the processing of transport rules for a variety of purposes like selective message encryption, application of email signatures, and so on.

The SMTP-based transport pipeline has existed in its current form for many years. It’s here that reply-all storms are detected and suppressed as distribution lists are expanded into individual addresses and sent on their way. If the transport service detects ten reply-all responses to messages sent to over 5,000 recipients within an hour, it clamps down on further responses and blocks them for four hours to allow the storm to calm.

Anyone who tries to send a reply-all response during that time receives a non-delivery notification (NDR) telling them “the conversation is busy with too many people” with advice about what to do next. Whether the recipient reads and understands what’s in the NDR is a debatable point.

At the Ignite 2019 conference, Microsoft reported that early versions of reply-all suppression worked well in their internal tenant. Now that the feature is released world-wide, Microsoft plans to gather data about how effective suppression is “in the wild” and use that telemetry to tweak how it works. For instance, an eight-hour delay might work better than four hours. Microsoft is also considering if they can generate reports to help tenant administrators understand how reply-all storms happen.

Reply-All Storms Only Happen in Large Organizations

The important thing here is that this feature is only for large Office 365 tenants where distribution lists of more than 5,000 recipients are found. Microsoft is one such organization, but the funny thing is that since 2013 Microsoft has been telling customers to move communications away from distribution lists to Yammer. Obviously, even in the Year of Yammer, that advice isn’t always heeded inside Microsoft.

Recently Microsoft increased the maximum number of members in a team from 5,000 to 10,000, which begs the question of whether the equivalent of an email reply-all storm can occur in Teams. The answer is “no,” because a single copy of a message exists in the Teams message store (chats or channel conversations). It’s still possible to over-communicate with Teams (like posting a message to 50 channels at one time), but each conversation is a separate thread.

Not for Exchange 2019

Microsoft has stayed quiet on whether reply-all suppression will ever be found in an on-premises Exchange server. I see little prospect of the technology being retrofitted to Exchange 2016 and not much chance of it making it to Exchange 2019 or whatever the next iteration of Exchange will be for the on-premises market, what little remains of it.

Suppression Active Now

Tenant administrators don’t have to do anything to activate reply-all suppression as it’s now part of the Exchange Online transport service. The first time you might know about suppression is when users receive NDRs telling them of busy conversations. But that won’t happen unless your organization uses mega distribution lists. And it won’t happen if products like Teams or Yammer have supplanted distribution lists. Which they haven’t, except in the minds of some over-active marketeers.

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Tony Redmond has written thousands of articles about Microsoft technology since 1996. He covers Office 365 and associated technologies for Petri.com and is also the lead author for the Office 365 for IT Pros eBook.