Mail servers have a bunch of tooling for routing, retries, queue management etc. that already exists. If you do that via HTTP you end up having to reinvent a full message broker stack just for the sake of using a different protocol that lacks the abundance of components you can just drop in to do things like reflection (mailing lists), forwarding, filtering, retrieval, deletion, access control that you get for free with a mail server.

Maybe if we that entire ecosystem existed on top of HTTP it'd have been ok, but the actual transport is the tiniest little uninteresting sliver of what we got by using a mail server for queuing.

With respect to the DNS servers, the service discovery was for addressing, so we/a client component could e.g. e-mail "[email protected]" or similar. The SMTP updated one was not the service discovery one, but feeding our customer facing DNS infrastructure.

There's several ways of doing service discovery for HTTP services like this in an enterprise, using 3xx replies to redirect if you move the service. The simplest is to just use a URI authority that you can change as needed. The next simplest is to publish URI RRs in DNS. The next simplest is to use the .well-known URI local-part namespace to include information for client-side routing. Granted, the last two are recent developments.

Or we could just use DNS A or CNAME records and not have to do any of those, the way e.g. CoreDNS w/k8s or ldapdns on top of openldap, and similar does.

There are use cases for more complicated service discovery, especially discovery of external services, but for the scenario I described, which in involved discovery within our own network this all just would have added extra complexity for no benefit that would have made any difference to us.

Note that service discovery was not listed as one of the reasons I think http doesn't solve the issues that made us choose a mail server back then..the exact same service discovery mechanism would have worked just fine for http too. But we'd still have had to build all the message broker code from scratch instead of picking and choosing from pre-existing components.

I'm not saying that you shouldn't have used SMTP. It really is appropriate for this sort of thing. If you wanted an HTTP interface you might still make it inject messages into SMTP, with HTTP being only used for a browser UI, say, or with the HTTP server as a proxy for some reason.

My objection had been to LDAP. LDAP is too heavy-weight by comparison to HTTP w/ a JSON API.