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.References:Appian Documentation - Expression Language Syntax