OnRETR is a TIdPOP3ServerMessageNumberEvent property that represents the event handler signalled when a threaded client connection performs the POP3 RETR command verb. 

OnRETR allows the server to retrieve the specified message number from the server mail store. OnRETR must write the RFC-compliant contents of the message to the connection for the POP3 client. OnRETR is responsible for writing the initial +OK response that includes the message size, the contents of the RFC message, and the terminating '.' for the retrieved message. 

The message number cannot refer to a message marked for deletion, or a negative POP3 response should be returned from the event handler. OnRETR is not triggered when TIdPOP3ServerThread.State contains a value other than Trans. Negative POP3 responses are written by the RETR command handler when TIdPOP3ServerThread.State is invalid, or the OnRETR event handler has not been assigned.

TIdPOP3ServerMessageNumberEvent TIdPOP3ServerThread.State TIdCommandHandlers TIdCommandHandler TIdCommand