Email examples for a newbie

Hi,

Can someone point me towards or send me an example of smtp proxiing(or
let me know that nginx will not do it)?

Scenario:

A single server running nginx on port 25. Emails to [email protected] get
delivered to server 192.168.0.5.
Emails to [email protected] get delivered to server 192.168.0.4

Thanks,

Jimmy