@toby3d (Wow, a lot of mail-related content in my feed lately... See https://mastodon.ml/@cyrmax/112830470535880679)

I don't think there are any open-source solutions, because part of the problem is not technical: you'd have to fight against people who think you're sending spam. Services like Mailgun, Mailchimp, AWS SES take care of that, and in exchange they get the right to terminate your account if you really do send spam. (I'm not saying you did! They can also terminate you because it's Friday or their left heel is itchy or whatever.)

If you want to go the self-hosting route, you'd have to set up an SMTP server like Postfix or Exim, or a complete email package like Maddy (plus a few dozen other little things). This can be simplified somewhat by projects such as Mail-in-a-box or Mailcow. Then actually sending an email becomes so simple you can do it by hand, and you get a new problem: getting your mail delivered to the recipient's inbox.

(I don't run an email server, so this answer is based on my research in preparation of setting one up.)