When testing the validity of an email address in Appian, you should consider various scenarios to ensure robust validation. This includes checking for the presence of the @-character, which is essential for a valid email format (A), testing how the expression handles null values (B), and verifying that a correctly formatted email address is validated as such (C). The availability of the mail server (D) is not relevant to the format validation of an email address within an expression.
Reference: Appian Documentation - Expression Language Syntax