To prevent abuse, email providers need a system to rate and sometimes block senders with bad intentions. Your reputation basically means whether you’re a spammer or not.
Spam complaints (when somebody marks your email as spam) and high unsubscribe rate affect your reputation.
ISPs are cautious with low-reputation senders because they can bring on problems with the system and the other users as well. We know you mean well, but you should be careful not to look like a spammer with the gazillion emails you shoot out every day.
Keep in mind that providers such as Mandrill (Mail Chimp) have hourly quotas for the number of emails that can go through. Quotas are automatically calculated on your reputation (check yours from your dashboard). The lower it is, the fewer emails per hour you can send.
So if you try to email everybody on your list at once, emails will only get stuck in a sending queue and then get deleted (in Mandrill it happens after 48 hours).
А bad reputation is hard to fix and can even lead to a ban. Yes, it happens and you have to switch platforms, along with other issues.
Good reputation, on the other hand, means your emails are sent fast and your hourly quota moves up over time. What’s available to you only gets better.