Lots of small organizations, especially non-profits, want to send out email newsletters to members and supporters. Mailchimp is probably the best-known email service, but it can be daunting for some small organizations - especially if the newsletters are being sent by volunteers who don't have the time to learn how to use all the options of a system like Mailchimp.
Buttondown is a great alternative.
Non-profits can email Buttondown's creator, Justin, to request a non-profit discount. (Both Buttondown and Mailchimp offer free levels, but they may not be enough for some organizations.)
I get a lot of great advice from the community on Ask MetaFilter, and several members recommended Buttondown for small organizations.