Note: lookup table == database in postfixese 
Lookup tables store information in (key,value) pairs
Some databases only need the key. These are lookup lists rather than lookup tables. We only check a key for existence (if there's a value associated, it is discarded).
A lookup table is defined like:
type:table
Valid types include hash (BrekeleyDB, generate/update with postmap), ldap, mysql, pgsql, regexp, pcre, etc.