How to leverage HTTP upstream features

[Charles Orth]

Hi Gurus,

I'm new to nginx...
I would like to define upstream handler in HTTP without a HTTP server 
listener. From SMTP/POP3/IMAP I would like
to use the upstream handler for my http endpoint. Thus requiring 
http_upstream initialize, create a request, hand the request off to 
upstream for processing and finally having HTTP upstream handler return 
the entire response to my handler.
I haven't found any examples or patches where I can leverage HTTP 
upstream from Mail service perspective.
Does anyone have a suggestion or an example?

Thanks
Charles