it's an email service provider for campaigns and other email spam with a recognizable brand, let's not imagine that this is beneficial beyond helping companies market stuff to people. It's like a meta-business enhancement service. Happy for the founders, hope they do some good with the new payout - but I think you are giving them a bit too much credit. Sure, kick back 100M to your old school district, maybe another 250M to BLM or to Math for America to strike off all of your philanthropic efforts in one go, curate some plebeian sympathies and smile for the cameras but at the end of the day, making it easy to bombard people with endless email campaigns isn't exactly solving global problems
EDIT: I reject hero worship vigorously - Mailchimp did a thing, it made money, and another entity that did a thing and made money that makes MORE money decided to absorb it and the cash payout was so good that investors/founders of MC decided to say fuck it, I'm done. Take my chips, gimme the loot. 12 billion dollars is generational wealth, but I mean I just can't/won't/don't respect people that have enabled spam or a business model that encourages bombarding people with BS. Same thing with IAP - I don't respect people who work for Apple because there was a layered hierarchy of priorities that said "we want to generate revenue at the expense of the people that trust us with their consumer electronics"
I just intrinsically cannot respect people that decide to profit off of wasting people's time, the only commodity you can't generate more of. Instead of craftsmanship we have psychopathic operators that try to fine tune all the ways to extract money from others and even develop meta-endeavors where people pay others to execute activities to get end-user observers ('users' in modern web dev parlance, for the JS devs) to waste time/spend money, for net negative expected value to themselves and others. It's super sordid and I just can't endorse it. Whatever.
In 25 years these guys will probably pull a Paul Allen and then start funding brain research or something after realizing that all the money in the world can't buy time and more life.