Get a ton of IP space while it was cheap and available several decades ago (mailchimp has multiple /17s and is old enough to drink).<p>Mailgun was a Rackspace product and had access to basically infinite amounts of Rackspace IPs.<p>Mailjet and Mailgun are both owned by a marketing company.<p>Other than that it's just a message queue exercise.
It's a front-end application around a SMTP mail server.
<a href="https://datacadamia.com/marketing/email/server" rel="nofollow">https://datacadamia.com/marketing/email/server</a>
You take your favorite language and you build your own.
That's hard ... ip control (they may be blacklisted), high availability, bounce and other features requires a lot of work.
The easy way is to create your application around a transactional email service. You pay for the email but not for the application :)