https://www.leafnode.org/ https://www.leafnode.org/ is decent if you just need a small instance. https://james.apache.org/ https://james.apache.org/ for more en

https://www.leafnode.org/ https://www.leafnode.org/ is decent if you just need a small instance. https://james.apache.org/ https://james.apache.org/ for more entreprisey stuff. And the old but reliable https://www.eyrie.org/~eagle/software/inn/ https://www.eyrie.org/~eagle/software/inn/