SmtpActions constants are used in different Events to define if action will be allowed by connected user, or denied. For example, when connected user sends RCPT TO: and MailTo event is fired, inside event you can set Action variable to Deny and connected user will receive an error.

SilentAllow and SilentDeny are exactly the same as Allow and Deny, except wodSmtpServer does not send any response back to the user. You should do it manually using User.Send method if you use one of these constants.