There's nothing extraordinary in the catalina.out that you've posted. Where are you seeing the "Too many files opened" message? What's in your /var/log/zimbra.log or /opt/zimbra/log/mailbox.log when the system dies? When you say 'dies' do you mean they just stop responding or they actually terminate?
There was a problem with kvoop, described in this
bug report but that's supposed to have been fixed. Could you be hitting the same (or similar) problem with attachments?