Another vote for migadu. I've used migadu for my family between two domains on their cheapest plan. I've been very happy with it. Their webclient while function

Another vote for migadu. I've used migadu for my family between two domains on their cheapest plan. I've been very happy with it. Their webclient while functional is a bit barebones so we use phone email + emclient on laptops with nextcloud handling contacts and calendar.