CentOs MySQL Support
You may want to use MySQL to store passwords of accounts, especially when you are working with virtual mailboxes.  One thing to note when you do compile Postfix is to make good use of the READMEs are they are very helpful. Be sure that the paths are correct for your distro.  Here you can see the changes necessary for CentOS.

This is in the README
make -f Makefile.init makefiles \
‘CCARGS=-DHAS_MYSQL -I/usr/local/mysql/include’ \
‘AUXLIBS=-L/usr/local/mysql/lib -lmysqlclient -lz -lm’

Modified
make -f Makefile.init makefiles \
‘CCARGS=-DHAS_MYSQL -I/usr/include/mysql -I/usr/include’ \
‘AUXLIBS=-L/usr/lib/mysql -L/usr/lib -lmysqlclient -lz -lm’

ldd `postconf -h daemon_directory`/smtpd
linux-gate.so.1 =>  (0×00940000)
libmysqlclient.so.15 => /usr/lib/mysql/libmysqlclient.so.15 (0×00748000)
libz.so.1 => /usr/lib/libz.so.1 (0x00dd7000)
libm.so.6 => /lib/libm.so.6 (0x00d41000)
libdb-4.3.so => /lib/libdb-4.3.so (0×00411000)
libnsl.so.1 => /lib/libnsl.so.1 (0x001a1000)
libresolv.so.2 => /lib/libresolv.so.2 (0x003fc000)
libc.so.6 => /lib/libc.so.6 (0x00bdc000)
libcrypt.so.1 => /lib/libcrypt.so.1 (0x001ba000)
libssl.so.6 => /lib/libssl.so.6 (0×00110000)
libcrypto.so.6 => /lib/libcrypto.so.6 (0x0050a000)
/lib/ld-linux.so.2 (0x00bb9000)
libpthread.so.0 => /lib/libpthread.so.0 (0x00d28000)
libgssapi_krb5.so.2 => /usr/lib/libgssapi_krb5.so.2 (0x0064d000)
libkrb5.so.3 => /usr/lib/libkrb5.so.3 (0x006b0000)
libcom_err.so.2 => /lib/libcom_err.so.2 (0×00157000)
libk5crypto.so.3 => /usr/lib/libk5crypto.so.3 (0×00688000)
libdl.so.2 => /lib/libdl.so.2 (0x00d22000)
libkrb5support.so.0 => /usr/lib/libkrb5support.so.0 (0x0067d000)
libkeyutils.so.1 => /lib/libkeyutils.so.1 (0x00df8000)
libselinux.so.1 => /lib/libselinux.so.1 (0x00db2000)
libsepol.so.1 => /lib/libsepol.so.1 (0x00d6a000)