Whoops…. I clicked reply-all…. have you ever accidentally hit the reply-all on an e-mail with a huge number of recipients?
Grrrrr…..Why did that person do that…Have you been the recipient of a reply-all e-mail which is just time wasting, or wholly inappropriate for the distribution?
Large numbers of organisations will have their Exchange distribution lists configured optimally. They are configured with appropriate permissions so that only limited people can send to them. Sometimes this is planned, but sometimes this is reactive configuration as the result of someone misusing this Reply-All function. In most instances this will not be malicious, but the result can be quite severe with disruption to normal activities with respect to mail-flow and bandwidth. Exchange Online has several features designed to prevent Reply-All storms, such as allowed sender lists, and recipient limits. However this can still not be configured correctly.
Reply-All Storm Protection in Exchange Online will detect when a Reply-All storm is happening and will introduce a temporary block for several hours. This means that users are blocked from replying to everyone in a thread. This cool-down period is designed to provide more time to reduce the enthusiasm to reply-all to the original thread. The user is presented with an NDR effectively telling them to not reply to all. This process is designed to curb the potential reply-all storm before it begins or gains any momentum, therefore keeping Exchange Online Transport running as expected.
This feature is due in all Exchange Online environments from April 2020.