On-demand mail relay is the configuration settings of an email server that does not have a continuous connection to the Internet, also known as a dynamic protocol. An email server with a constant connection to the Internet has a static Internet protocol or IP address. This service uses the SMTP ESMTP profile on port 366. With this setup, all new mail for the email server is held by the Internet protocol (IP) service provider. When the email server connects, the messages received are sent and any new messages are delivered.
When there is a dynamic IP address, the IP address changes with each connection. In order to reliably send/receive email messages, an on-demand mail relay service is required. On-demand mail relay is also known as ODMR, authenticated turn or ATRN.
Within the email server software, the email administrator defines the mail delivery rules and protocols. The documentation or user manual may make reference to an ATRN client, take note as this is the same as an ODMR client. Multiple downloads can be arranged and message distribution scheduled to meet your user communities requirements within this function.
A good illustration of this concept and the different between on-demand mail relay and continuous relay is the post office. If you have on-demand mail relay, your mail is held at the post office until you request it. Then all the mail addressed to you is delivered and any mail that you want to send to others in picked up and distributed. Customers with a continuous relay have a courier come to the door with every mail item received and have another one come to take each item of mail for delivery.
Efficiency of computer resources is harnessed by setting up the appropriate frequency for this type of service. An on-demand mail relay provides the best balance of managing user needs and system resources for the vast majority of email users.
To send and receive email, the server is programmed to connect at standard, pre-configured intervals. The email server connects to the IP service provider, using a specific port and IP address. An authentication process is followed; usually the exchange of security certificates and then the mail is requested. In this scenario, the email server is the client and the IP provider is the server. Once the mail is received and all the outgoing mail is sent, the connection closes and the client email server become the primary server and distributes the mail received to the end users email account.
This type of connection is very common with consumer email accounts, small business running their own email services and large corporations looking to manage system resources. The costs are lower than a static IP address and the inconvenient to the end user is minimal.