I know this was just a crude crude hack but I once implemented something akin to what you describe using a bunch of domains on Namecheap and their "wildcard red

I know this was just a crude crude hack but I once implemented something akin to what you describe using a bunch of domains on Namecheap and their "wildcard redirect" feature, where they would redirect allusernames@foo.bar (a catch all), forwarding all email, per domain, out of all places to a self-hosted Zulip instance, so each domain would forward to a Zulip channel (each Zulip channel gets an email address you can activate and forward messages to). Worked well. Pretty neat. Of course in this particular case only cared for the incoming mails, which may or may not be your case