Mailscanner + postfix: dealing with huge loads on distributed mailserver setup

[l at avc.su]

Hi All.

We've got 2 servers with MailScanner+Mailwatch, using Postfix as MTA.
Recently we've experienced huge incoming load from distributed locations 
(our marketing team fired up large newsletter and our corporate server 
got about 10K "auto-reply" mails in just 20 minutes), and our 'hold' 
queue got filled up with mails. It caused big delay in recieving 
legitimate mails, and MailScanner (15 children max.) was buisy with them 
for about an hour.

Is there any way to prevent Postfix from recieving any new mail if there 
is a clog in Hold queue? For example, if there are more than 500 
messages, don't recieve new mail and respond with 421 code so clients 
would go to another mail server?
I haven't found any solid measures against it, and I'm considering to 
write own policy-server for Postfix. It's complicated task (and I'm lack 
of coding skills), and I want to be sure that I haven't missed vanilla 
setting for this.

So, basically, there two questions: is there some sort of failswitch  to 
prevent Postfix to get new connections if there is a clog in queue, and 
how do you deal with huge mailing loads?

Thank you.