Exchange Online – Reply to All mail storm protection is coming

It has been announced few months back during the Microsoft Ignite 2019 conference and it is now coming to Exchange Online: the Reply to All mail storm protection.

We all get annoyed when people is starting to Reply All on message send to distribution list and this become a mail storm (the infamous ‘Me too’ or ‘Remove me from this’ situations).

The roll out of this ‘Reply All Mail Storm Protection’ feature is currently underway.

With it, Exchange Online will detect when such storm is occurring and will block any further reply and will send back NDR (Non Delivery Report)


The detection is based on 10 reply all-s to over 5000 recipients within 60 minutes and any further reply to this specific email thread/storm will be blocked for 4 hours.

This feature is likely going to benefit organizations with large distribution lists.

You should start updating your internal documentation and communicate to your end-users and support team to read the NDR messages they may get (Exchange Online has been producing clearer and understandable NDR for quite some time now).

While this feature will be really helpful it may back fired to an increase of support call when the storm protection is kicking in.