Multiple reverse proxied smtp servers

I run a number of websites for community organisations and charities. In recent times I have been gradually moving each of them into their own vm (under kvm). This week-end I have used nginx to reverse proxy the websites and this works well.

Some of the organisations run their own mail and / or listservers (Mailman). I would like to run these mailservers within the same vm for ease of administration and portability.

I note nginx can reverse proxy smtp but could not work out how to do this for more than one site. Is this possible? Is there a howto?

David

P.S. Sorry if this is a newby question answered somewhere.