After setting up postfix to use mysql as lookup backend for alias or virtual or whatever table, everything seems ok. Mails are delivered.
But sometimes strange "Postfix SMTP server: errors from " e-mails are received. Usually more than one in the same second, then there can be no errors for days or weeks. Then they appear again. There is a "Temporary lookup failure" or "queue file write error" cited in those error e-mails but everything seems to work ok again. Even those e-mails that get error on first try get delivered on second try some minutes later.
One thing to look out for is
Sometimes squid stops working and "WARNING! Your cache is running out of filedescriptors" is logged in cache.log. Restart usually resolves that problem. Temporarily though.
This article is based on http://www.cyberciti.biz/faq/squid-proxy-server-running-out-filedescriptors/ adding some Gentoo specific bits.
Squid, as a web cache should, holds a lot of files from internet on a local hard disk. It means that a lot of files are open at the same time. On busy servers this amount grows till the limit imposed by the operating system. Then no more files can be opened and squid seemingly dies.