enable spamassassin?

[rbtux]

why would you think that? IMHO it's just the oposite in fact:

with pre-queue filter postfix stops accepting mails when the amavis proxies can't handle the load. (It returns a 4xx error). So the mails can be processed by your second mta. With post-queue filter it fills your queue until the spool directory quota is reached (or some restrictions are raised). Mails are delivered FIFO.

When all your mx are under attack you have a problem anyway... and it would rather have no mails accepted as a queue containing tousands of mails...