If you are willing to run your own, have a look at. https://poste.io/ https://poste.io/ I'm currently evaluating and running this myself for about 2 Months now

If you are willing to run your own, have a look at. https://poste.io/ https://poste.io/ I'm currently evaluating and running this myself for about 2 Months now. Really working great so far. The guys have very quick response to error reporting. Everything run via docker on a aws small instance. An completely free version could also be: https://mailu.io/ https://mailu.io/