Internet Direct (Indy) Version 9.0.2 Beta
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property OnCommandEXPN: TBasicHandler;
Application must assign a procedure to the OnCommandEXPN event handler to allow responding to the event notification. When OnCommandEXPN is unassigned, the server will send the text in Messages.ErrorReply to the client connection.
An SMTP server may choose to disable EXPN for security reasons. EXPN has been used by spammers to harvest email addresses. An SMTP server should also use some type of authentication mechanism, like OnCommandAUTH, to prevent this type of abuse.
When EXPN is enabled, the implementation must not indicate an email address as verified when it has not been tested.