Unfortunately, SMTP proxy support is not in rspamd yet (maybe I'll add
it as GSoC project). To clarify more, I'd say that there is *some* code
but it is not of the production quality.
Another option is to use OpenSMTPD filters framework. It seems to be
very clear and straightforward to make an HTTP request to rspamd and
parse the reply. But this work is not done as well.
--
Vsevolod Stakhov