Develop SMTP/POP/IMAP proxy with custom authentication

I need someone with deep expertise in working of SMTP/POP and IMAP protocols and undertsanding of SMTP/POP proxies. The project is simply to create a stable SMTP proxy which can handle large loads and connections. The proxy has a simple purpose of doing custom authentication. SO what I want is to intercept all the incoming authentication requests. The incoming password will hav ethe following format - 6 digit PIN+OriginalMailPassword The filter will split the password, get the 6 digit pin authenticated by my server. This will be done by an already available and ready webservice call(Https call..no SOAP used) and then send the UN and OriginalMailPassword to the original MTA. In other words for most of the cases it will just work as a standard forwarding agent except for Authentication requests. I am ok with modifying existing Open SOurce Tools like Postfix.