SMTP-POP3 proxy

Hi there,

I'm asked to develop a "let's call it like that" SMTP-POP3 proxy.
If you have Norton products, you can see a sort of proxy for verifying emails (ie the mail client connects to the Norton proxy which is connected to the real email server)

Well .. I know you've understood since the first line .. anyway ..
My problem is that I have to develop it on multiple platforms ... let say ... Windows - Mac OS 7, 9 and X - Linux (RedHat, SlackW, ...), SunOS, HP-UX, AIX, etc ...

I know that for Windows I have the MAPI and it's no big deal on this platform :p
But I don't know MacOS and Linux possibilities in term of libraries ... :(

Finally the question : On Mac and Unix systems, are there libraries that eases such a development or am i supposed to developped a complete SMTP-POP3 command interpretor (client-server proxy)... ?

Thanks for your patience .. and sorry for my bad english :)

Cliff