Requirement: selector.operator.is

IS expressions are of the form: identifier IS [NOT] NULL The IS NULL form tests for a null header field value or a missing property value. The IS NOT NULL form tests for the existence of a non-null header field value or property value The IS NULL and IS NOT NULL operators convert an unknown header or property value into the respective TRUE and FALSE values.

See:

• Section 3.8.1.1, Message Selector Syntax
• Section 3.8.1.2, Null Values