I would love to use MailChimp, but it was price prohibitive when I first got started. The site was using Mailgun when I acquired it, and I stuck with it after evaluating a BUNCH of options when I got up to around 30-50k users.

I don't recall what MailChimp's pricing was back then, but 50k users would be $640/mo today (vs ~$80 on Mailgun). I'm currently north of 300k users, which is up into the "contact us for pricing" range. It just doesn't make sense for a site with revenue sub-$10k (like, well sub-$10k).

I could've dropped that price by spending a shitload of time deactivating contacts and trying to keep my active users count lower, but email just isn't that profitable for me. I wouldn't say I'm great at email or attribution, but I estimate I drive ~$250-500/mo in sales through email (up from $0 when I started).

The other points are good ones -- even on Mailgun, I should've had different subdomains set up for IntroCave for transactional/campaign/newsletter blasts. I got "good enough" results doing it the lazy way, though. I still wouldn't have remembered to migrate the bounces, and this might have actually increased my bounce rate. Since bounce lists are per-domain and not per-account, a signup bounce wouldn't necessarily have prevented me from sending transactional stuff later (I've since added a bounced flag to my user model which would catch this).

(edit: actually the first point is pretty good. i disagree with the rest. list was mine, domain was warmed with a week of email before the ToS blast, and I manually ramped up volume on the ToS blast for the first 30 hours or so while keeping an eye on deliverability. This was entirely my fault, so I'm not sure what you mean by blame... I'm not happy with the customer support response, but the title of the post should make it pretty clear where the blame lies--me!)