The only bit I see that talks about something similar is, "Salmon is designed to sit _behind a traditional mail server_ in the same way a web application sits behind Apache or Nginx", emphasis is mine.
It would be nice if the documentation had some examples of how to do that, though. All it says is this:
> Salmon is best deployed behind another mailserver such as Postfix or Sendmail - much in the same way as you host a WSGI application behind Apache or Nginx.
The only bit I see that talks about something similar is, "Salmon is designed to sit _behind a traditional mail server_ in the same way a web application sits behind Apache or Nginx", emphasis is mine.