Someone apparently changed some settings in the EpicGames organization on GitHub, so that everyone in the organization is automatically subscribed to all new repositories (including forks).
In the past 3 hours, there have been 3 forks of the main UnrealEngine repository:
…and about 125000 people have been subscribed to all changes in each of those forks…
So, someone at Epic accidentally sent out over 375000 emails. I expect that at least a million emails have been sent out in 24 hours.