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ODMR
On-Demand Mail Relay (ODMR) facilitates mail delivery by allowing remote servers to retrieve queued email messages as needed, rather than relying solely on traditional push-based SMTP delivery. This enables efficient mail forwarding in scenarios where recipient servers have intermittent connectivity or are located behind firewalls, enhancing email flexibility for ISPs and mobile users..
On-Demand Mail Relay (ODMR), sometimes referred to as ATRN SMTP, is defined under RFC 2645 and is designed to optimize message delivery particularly in scenarios where servers are dynamically connected, such as dial-up or mobile email servers. It modifies the traditional client-server SMTP model by allowing the receiving server to initiate the collection of its emails when it is online, effectively flipping the communication direction.
ODMR primarily works by having the client connect to the ODMR-enabled server, authenticate itself, and then request pending email messages using the ATRN command. This command enables the server to transfer messages in the same session without waiting for the sender to establish a separate SMTP connection, thereby enhancing efficiency and reducing connection delays.
This protocol is important for ISPs and email hosting providers supporting customers with intermittent connectivity. Its design helps manage mail queues reliably, reduces unnecessary network retries, and ensures that queued messages are batched and delivered promptly upon client request.