Email storms and nuclear meltdowns have the same rough physics.
If 1% of emails are blind replies from people out of the office, but are tracked by the originating sender (not the DL) since OOO messages in Exchange get sent to the DL (as well as the originating sender) and soon enough you have Exchange sending a new OOO email per reply from a new reply-er. Your typical company the size of Microsoft is going to have probably... 3-5k people out of the office at any one given time (the lingo is "OOF", or "out of facility")
OOOs can also trigger another OOO, which can in turn cause a new OOO to spawn.
Soon, you've got OOOs reply-all'ing to OOOs and drenching a DL... and an exchange forest.
If 1% of emails are blind replies from people out of the office, but are tracked by the originating sender (not the DL) since OOO messages in Exchange get sent to the DL (as well as the originating sender) and soon enough you have Exchange sending a new OOO email per reply from a new reply-er. Your typical company the size of Microsoft is going to have probably... 3-5k people out of the office at any one given time (the lingo is "OOF", or "out of facility")
OOOs can also trigger another OOO, which can in turn cause a new OOO to spawn.
Soon, you've got OOOs reply-all'ing to OOOs and drenching a DL... and an exchange forest.