reverse proxy SMTP - How distinguish MUA and MTA

[dethegeek]

Hi

Still building a nginx reverse proxy for my mail servers. Thanks to the
community, I now have a secure connection between nginx and my backend mail
server. 

POP and IMAP are working well, from a MUA to my server.

I'm wondering how nginx can manage SMTP coonnections as it is used by both
MUA and MTA.

A MUA must authenticate before sending mails; and my http_auth backend is
able to authenticate users.  More precisely, the authentication backend
answers a server (auth-server / auth-port) depending on the domain of the
destination email address.

Now I guess I have to accept incoming emails without authentication if the
client is a MTA, but I don't find a obvious way to distingish a MUA and a
MTA and let mu auth backend behave depending on that.

How to achieve that ?

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,259384,259384#msg-259384