but how did you achieve it? what's your delivery rate like? do you find email providers are often marking messages as spam?
It's actually a really simple system; the site you see is just a classic PHP site that lets you make lists, and write newsletters, and handles signups (PHP just emails an agree template to someone each time they signup using
mail())
There's a backend SQL database with all the lists, newsletters and signups. When you mark a newsletter for sending, a copy of that newsletter is made for each subscriber in a big outbox table. Then there's a super small NodeJS script that runs in the background and checks the outbox table for pending messages, it goes through each one, sending it via Postfix (I think) and flags it as sent.
I set it up over a year ago and more or less have not had to touch it since so I actually don't remember exactly, but I think I just googled "How to host an Ubuntu mail server and followed the guide"

It was tricky getting the emails to sign themselves properly so they don't get flagged as spam.
mail-tester.com gives you a lot of good info about how spam-ish your emails are. This site is hosted by a small company, so they were willing to assign the IP address to be "owned" by melonland.net meaning its much more trusted than a generic VPS would be. I think I watched all the harry potter films back to back while setting all that up.
As for your questions, I frankly dont care

As far as I can see newsletters always send, they don't get dropped on my end. I have no idea if emails get marked as spam, it would depend on the provider and how the person has trained there spam filter. Again not really my issue - if someone wants to read my newsletter, they better make sure they can read it

As a whole I just wanted to be able to send newsletters with HTML and no weird restrictions, and no service was offering that for a fair price that made sense for an indie site owner. So I was gonna make the newsletter service anyway, putting in a little extra to make it usable by other melonlanders was a bit more time, but not that much more, and I enjoy all the silly newsletter people make.
I don't think an overload of people will be an issue, its actually a very underused feature; but if it overloads some day that could be fun and can be dealt with at the time.
I'm definitely not a master at anything, but I am good at connecting things and not worrying about them breaking
