OnCommandEXPN is a TBasicHandler event handler property that represents the event handler signalled when executing the SMTP EXPN command verb using CommandHandlers. 

OnCommandEXPN allows the server to implement the processing needed to expand the email or mailing list address provided in the TIdCommand in ASender a return the email addresses that are members of the list. 

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.

TBasicHandler OnCommandAUTH Messages