If your Zimbra system gets targeted by spammers, you'll notice that a spammer's email server can open up a large number of simultaneous connections to Zimbra's Postfix.
Most of these connections will fail, often because the recipients don't actually exist on the system. But, these connections still use resources.
So, we have for years on our other Postfix mail servers been taking advantage of two Postfix configuration settings that have reduced this problem significantly. We have now updated our Zimbra installations with the same settings, so I thought I would pass them on.
The two settings we add to main.cf are:
smtpd_soft_error_limit = 2
smtpd_hard_error_limit = 3
We do this by becoming the zimbra user and then running:
postconf -e 'smtpd_hard_error_limit = 3'
postconf -e 'smtpd_soft_error_limit = 2'
We then restart Postfix to implement the changes.
Documentation is here:
Hope that helps,