Even organizations I would expect to get DKIM/SPF/DMARC right frequently don't. I don't know how many "Hey, I see your emails have DKIM/SPF problems, here is how to fix it," messages I've sent that nobody ever followed-through on.
So startups opt for the monthly plan, and then in order to "get their money's worth", they send more emails than they might otherwise.
I sincerely wonder if MailChimp's pricing structure ends up causing there to be substantially more spam in the world. I don't want to hassle my users all the time, so I just pause my monthly service and unpause it once or twice a year.
But really I'd like to send 4-6 emails per year. And with MailChimp, I can either pay exorbitant per-delivery costs or pay for several months of subscription where I'm not sending any emails. Neither option seems great. Once email marketing becomes more of a focus for my company, I will look seriously at competitors to see if they have offerings that are structured differently.
Here's what I've found: Say you're generating a new list from user entered email addresses, supplied by users with a form on your website. After regex for validating the "shape" of addresses, and also making the user enter their email twice (which I'm finding doesn't actually help your correct email rate, all it does is reduce the number of signups), a full 6% of gmail addresses you receive will still be invalid, and an additional 3% will be trash ('email@example.com'). For non-gmail domains it's more like 20% bounce, 20% go to yahoo/outlook/hotmail black holes w/o bouncing, and you end up with about 60% delivery of your double opt in mail, of which about half actually bother to dig out of spam and verify. Awful! If you then went on to send one email per year when you have some big important event that your users actually signed up to hear about, I have some bad news for you. 30% of those emails will bounce, and you'll get on all the spam lists immediately. So, how do you get your bounce rate down? Simple! Send a welcome email - 30% of those will bounce, as before, but then, for those that don't bounce, send another "how're you doing!" email once a week. Voila - after a month, your bounce rate is down to 6%, your users hate you because you send trash all the time, but hey, from google, microsoft, and veriozon's point of view you're a great member of the email community, look how low your bounce rate is getting to be!
If you run an email server, please do the right thing for your legitimate marketers and ignore bounce rate - the extra 5% traffic to your smtp server is not going to burn it down. If you think high bounce rates indicate someone trying some sort of dictionary attack, well, sure - but this should generate a bounce rate that is near 100% - if you set your cutoff somewhere reasonable, say 50%, then you stop forcing me to have to send 5 emails to every signup!