An EBMK refers to any email announcement sent to an office listserv that starts off a chain reaction of boastful , fakely supportive or congratulatory 'reply all' emails to the entire department. EMBKs are initially sent by a single person to a group list and, in response, co-workers start responding to the entire group. At first just the narcissists or those who don't know their place in the hierarchy send reply all emails with fake and overly emotional "congratulations!!!", "thank you so much!!", "this made my day :) :) :)", etc. Then the higher-ranking members of the office try to one up each other with longer winded pontificatory reply all emails... these are interspersed with reply-alls from their subordinates and other sycophants, often with emojis or an awkward number of exclamation points. Eventually the email chain reaches a critical mass where average and even fringe group-members feel peer pressure to reply all to the group with similarly disingenuous expressions of emotion, or else it looks like they are not supportive.

EBMK is adapted from the Russian nuclear reactor type RBMK ("reaktor bolshoy moshchnosti kanalnyy", or high-power channel-type reactor) which underwent a chain reaction and blew up in Chernobyl, 1986, igniting an ever expanding shitstorm that negatively affected all nearby.
"Ah crap, my boss just sent an ebmk to the group about his new baby and everyone is replying to the group, now I have to reply also or I'll look bad..."
by Sqeezle May 28, 2020
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