You write an expression that checks the validity of an email address. Which three scenarios should you configure as test cases? (Choose three.)

When testing a validation expression, it is crucial to cover a comprehensive range of scenarios to ensure its robustness. The best practice is to include test cases for the "happy path" (a valid input), common failure modes (negative paths), and edge cases. A valid email address (C) confirms the expression works as expected for correct data. An invalid format, such as a missing "@" symbol (A), tests a specific, common negative scenario. A null value (B) tests an edge case to ensure the expression handles empty or non-existent inputs gracefully without causing an error.

D. The mail server is unavailable. This tests the availability of an external system, which is outside the scope of an expression designed to validate the syntactic format of an email address string.

Appian Documentation, "Create Test Cases for an Expression Rule": This document emphasizes the importance of comprehensive testing. It states, "We recommend creating test cases that cover a variety of scenarios, including edge cases, to ensure your expression rule behaves as you expect." This directly supports testing valid inputs (happy path), invalid inputs (negative path), and nulls (edge cases).

Appian Documentation, "Expression Rule Test Cases": This page details the mechanism for creating assertions where you provide test inputs and define the expected output. This process is designed to validate logic against different inputs, such as a correctly formatted string ("[email protected]"), a malformed string ("john.doeexample.com"), and a null value.