I use postfix, dovecot and snappymail. Very happy.
I already had a well-configured postfix (DKIM, DMARC) for web applications, so adding dovecot was easy. It's also really powerful, you can define who gets mailboxes in scripts. I tried going with providers but I couldn't get this kind of power.
Holy smokes, Vikunja is the best todo-app the world has ever seen. It's SO GOOD. It manages to be insanely powerful and have a LOT of features you normally need to buy some kind of super-enterprise-tier of a web app for, and it still doesn't feel bloated! I was never able to have a workable todo app because there were no categories, but now I run my entire life and family like a startup- need to get something organized- no problem, just create a shared project and that's that. It's like magic. I think it would be no problem to use it in a company as well. Thank you, Konrad!!!