This the kind of apologetics I can't stand. These things happen because businesses knowingly take actions that negatively affect a small percentage of their user base, but they just don't care if the percentage is small enough.
Software businesses enjoy extremely high gross margins, which means they can afford first-rate customer service. "Bad things happen" isn't an excuse.
But here's the deal, and this might be directly aimed at you, since you say you're in the field. As a home-user who works in the industry and with some interest for tech, I generally only get access to the free-tiers of services offered by players working at world-magnitude. Based on my experience with those services I may or may not advertise them to my friends, colleagues, employers. Especially to employers, because there's a good chance that if there's bad-blood between me and the service provider personally, I might be impaired in my professional activity.
This word-of-mouth type of adversiting is crucial to "2.0" companies, that function based on things such as scale, transparency, growth, reach, efficiency. There's also the different type of provider, the "old business" world, with more business-y and less tech-y practices, such as "call us for a quote" deals, "license per year per seat pe server core", etc. Dealing with them often times involves whole departments (legal + technical) with specific training and paid-for support channels.
If you release and roll perpetuum-beta services and software ("2.0" practice), build your brand on word-of-mouth advertising, on try-for-free honeypots for hobbyists (also "2.0"), don't act "old business" if it comes down to support for a puny user and don't push the "well, it was free, what would you expect?" button. The whole deal of your "we are awesome and scale as opposed to <brand that existed for more than 20 years and sucks just because of that>" is the fact that your machinery doesn't do politics and doesn't discriminate between your users based on estimated pocket girth. It's useless if your solution elegantly "scales" to billions of users, if your business can't secure and treat with dignity the first, lonely user.
Which is exactly what MailChimp failed to do, and what makes this an issue worth the treatment the OP has given it.
Shit happens from time to time in business and the normal approach is you connect and discuss by whatever SLAs you align to. You don't scream publicly sullying reputations unless you've given resolution a reasonable shot. Waiting less than 1 business day doesn't seem like giving resolution a reasonable shot. If it were my business, I'd fire this poster as a client.