If you can't compile Postfix because the file "db.h" isn't found, then you MUST install the Berkeley DB development package (name: db???-devel-???) that matches your system library. You can find out what is installed with the rpm command. For example:
$ rpm -qf /usr/lib/libdb.so db4-4.3.29-2
This means that you need to install db4-devel-4.3.29-2 (on some systems, specify "rpm -qf /lib/libdb.so" instead).
DO NOT download some Berkeley DB version from the network. Every Postfix program will dump core when it is built with a different Berkeley DB version than the version that is used by the system library routines. See the DB_README file for further information.
On RedHat Linux 7.1 and later procmail no longer has permission to write the mail spool directory. Workaround:
# chmod 1777 /var/spool/mail
LINUX syslogd uses synchronous writes by default. Because of this, syslogd can actually use more system resources than Postfix. To avoid such badness, disable synchronous mail logfile writes by editing /etc/syslog.conf and by prepending a - to the logfile name:
/etc/syslog.conf: mail.* -/var/log/mail.log
Send a "kill -HUP" to the syslogd to make the change effective.