ODMR works in the following way: to check their inbound mail, "customers" issue codice_3 and codice_4 commands to the ODMR service, followed by codice_1 command (Authenticated TuRN); after a successful codice_1 command the ODMR server begins to act as an SMTP client and starts to send all messages directed to the customer's domain (or domains) using SMTP protocol, via the same TCP connection. Since connections are initiated by customers, this protocol works well for machines having dynamically assigned IPs and/or are firewall protected (i.e. your ISP may block incoming connection on port 25 or may use NAT). Only the ODMR server, referred as the "provider", must listen to SMTP connections on a fixed IP address.