Ask HN: Which email marketing software should I use?

I run a mobile app company and I've decided to start collecting emails for cross promotion of my apps. I've used mailchimp in the past, and I have no issues with it, but I will likely be passing their free tier soon so I thought I should do some more research. I perfectly happy with suggestions that say, put your credit card into mailchimp and be done with it, if that's my best option

Generally speaking, I would hope/expect to have a bigger list that I send to 1-2 times a month at most. (as opposed to a smaller list that I emailed a couple times a week or something)

Price is likely the main factor, but ease of use is also key. I don't want to spend a ton of time coding my own X for this. Just get in, type out email, hit send. Autoresponders would be a bonus as well.

What would you use at X list size where X = :

25k? 500k? 5 mil?

Thanks so much.