[Qub3d]

You might like Migadu[0]. Its set up as more of a postmaster tool but does have webmail, and multiple accounts on a domain is a first-class feature. I found out about it from ddvault's recommendations[1].

[0]: https://www.migadu.com/

[1]: https://drewdevault.com/2020/06/19/Mail-service-provider-rec...