Although I can't yet understand all the details of how it happened, I know, at least, already, the general steps by which it happened.
But now, that I've resolved that, we have been being bombarded hard with tens of thousand of little SPAM mails (about 3,5 k, each) that were not being classified as SPAM and, thus, not being blocked.... :-(
And their volume caused a lot of problems, on it's own.
Yesterday I broke the record of the number of received mails, here, and today it seems I'm going to break it again.
The only good thing, in this, is that since 13h30m I managed to make changes on the SpamAssassin rules that, with a rule I wrote myself (it had to happen, soon or latter...) are allowing me to block, right now, most of the incomming SPAM at the inner inboud MailScanner...
The lessons to be learned from here are going to be incorporated on the new generation of mail servers I'm building right now.