I noticed this error during an ISPConfig on CentOS 5.2 install and realized I had installed postfix without MySQL support. After looking around for a bit I was unable to locate a yum repository that included a postfix package with MySQL support built in so I decided to download the source RPM and install by modifying the SPEC file. Below are step by step instructions of what to do.
1. Remove postfix: rpm -e –nodeps postfix
2. Download the src RPM here. Located in the following directory: /centos/5.2/os/SRPMS/
Example: wget http://mirror.mojohost.com/centos/5.2/os/SRPMS/postfix-2.3.3-2.src.rpm
3. Install source RPM: rpm -i postfix-2.3.3-2.src.rpm # Install the Source RPM
4. Edit File: cd /usr/src/redhat/SPECS
Edit postfix.spec: %define MYSQL 0 to %define MYSQL 1
5. Install Packages: yum install pcre-devel mysql-devel
6. Build The RPM: rpmbuild -ba postfix.spec (You will see lots of warnings, enjoy.)
7. Install new RPM: cd /usr/src/redhat/RPMS; rpm -i postfix-2.3.3-2.i386.rpm
That should do the trick. If you are like me and happened to be in the middle of installing ISPConfig then you will have a bunch of configuration changes to add back in for postfix.