+1 Been with Migadu since 2017 with no complaints. Lost some mails on an account due to a migration of sorts on their end though that was eventually restored after a ticket.

Michael and his crew are fantastic in progressively introducing new features that you may never find elsewhere (https://www.migadu.com/guides/index.html#features).

Your end users may hate the sparse webmail features though that's nothing that Thunderbird or Outlook may not resolve. HTH!