DCC - Postfix MTA mail hub implementation question

Vernon Schryver wrote:

>Besides, I'm not considering converting the DCC to a Postfix adjunct.  
>What other MTAs would want to use a filter whose interfaces is SMTP
>in and out?
>
That's the crux -- is there any DCC or general filtering protocol useful 
enough to be adopted by multiple MTAs? Or, will DCC have to conform to 
each MTA's filtering protocol as it has for sendmail? I think the latter 
is more likely.

One benchmark to look at may be virus checkers. Many of them offer to 
act as pseudo SMTP servers -- either in front of the real MTA or behind 
it in a filtering step in the SMTP chain. This is an example of a class 
of application with needs similar to DCC that has failed to foster a 
universal content filtering protocol (beyond SMTP itself).

I've chosen to let the other issues we were talking about (suitability 
of SMTP as an interface, where bottlenecks are, etc.) drop since the 
above is really the crux of the issue.