As someone who doesn't know much about MailChimp, the original question seems valid. A little more detailed answer (like the one by qeorge) would have helped me understand this better.
As you work at MailChimp, it would have helped if you would have given that detail.
Ben from MailChimp here. We invested a lot in abuse prevention, well before introducing any freemium plans. The problem ESPs like us face isn't so much the "evil" spam, but the "fuzzy" spam (from clueless marketers).
Probably shoulda brought that up in my blog post (it's usually the first question techies ask), but it's something our customers are pretty familiar with, so I left it out.
Thanks for your answer. I imagined so. But I could also think of some legitimate heavy-duty users who could threaten your profitability. Is there any fine print for those cases?
I don't think so. The cost of running something like Mailchimp isn't in having a couple of servers and some bandwidth, I imagine; the cost is in dealing with spammers, overzealous spam-fighters and people who'd rather hit "report as spam" than unsubscribe. Number of subscribers is not a bad metric.
ETA: Also see bjonathan's comment at http://news.ycombinator.com/item?id=2170619: Amazon, which probably offers something closer to "server and bandwidth", is much cheaper.
No fine-print. Our high volume plans for large email lists are not unlimited, but smaller lists are completely free to send as much as they want. In practice, you'll hit our anti-spam limits long before you hurt our profitability if you try to send lots of email to a smaller list.
As you work at MailChimp, it would have helped if you would have given that detail.