Next the mailSieveRuleSource attribute is checked for a Sieve filter that applies to this entry. If this attribute exists, it is parsed and stored at this point. The two possible forms for the value of this attribute are a single value that contains a complete Sieve script or multiple values where each value contains a piece of a Sieve script. The latter form is produced by the web filter construction interface. Special code is used to order the values and glue them together properly.
The use of the mailSieveRuleSource attribute specifically can be overridden by using the LDAP_FILTER MTA option.