Ask HN: What is the tech behind MailChimp/Sendgrid etc.?
What is the tech behind Mailchimp? What does it take to create a Mailchimp like email service? Can I just build it on Amazon SES
What that means is that you want full control over the actual mail servers to have influence and can adjust things. The easy route wouuld be SES, it would work but you basically have no control over anything.
When running your own mail servers things like IP repuation becomes important. You probably want multiple different IP subnets, so that if one or more IPs are getting blacklisted temporary you can use other IPs or subnets. This way you could also rotate through those IPs. Use the good IPs for good customers, IPs that got blacklisted in the past could be used for new users that are higher risk. Handling all those things is a constant hassle.