Action Mailbox as an incoming SMTP service?

Hi everyone! First post and I'm stuck understanding Action Mailbox. I'm looking at building... basically an SMTP service, but not quite.

I'd like users that signup:

• be given a user/pass/port like a regular email provider
• have my server/app capture incoming emails from users <- stuck here
• process the email (magic) then use Action Mailer (& 3rd party) to send email

I've looked at GoRails action mailbox tutorial, and seems all use cases involve people sending email to 1 specified address and processing based on the sender. It also seems the majority of SMTP SaaS companies provide the same kind of service.

But I need more of a traditional email provider setup. I haven't seen anyone build an incoming SMTP server with rails. Is it possible? Does rails have a mechanism to setup a service that acts like a Mail Submission Agent between the Mail User Agent and the Mail Transfer Agent? Or do I hook into a 3rd party (I'd prefer this). Or do I somehow hook into a Postfix setup or something similar?

Thanks for the advice!