It's a front-end application around a SMTP mail server.
https://datacadamia.com/marketing/email/server
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 :)
This is the worst answer I have read in a year. You should be gratefull someone taught you to wipe your butt. I think what you really meant to say was "I have no idea how email works either but I'm going to make you think you shouldn't try to figure it out because then you might become better than me."
I think what you really meant to say was “I’ve never built an ESP, scaled one, or maintained one but I’m going to criticize instead of giving any type of reasoned rebuttal to make you think I’m better than you without having to say anything of any substance.”
Congrats, outside of pure prejudice, this is the worst comment reply I’ve received on HN.
When you get your “5 Easy Steps for Building ESPs (#4 Will Shock You!)” blog post up I can’t wait to read it.
Mailgun was a Rackspace product and had access to basically infinite amounts of Rackspace IPs.
Mailjet and Mailgun are both owned by a marketing company.
Other than that it's just a message queue exercise.