Routing mail over transient connections
Sites that do not have permanent connections to the Internet, or to other servers on the Domino® network, can send and receive messages over a transient connection, such as a network dialup connection.
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For example, an organization that does not have a constant connection to the Internet might use a remote mail server at its ISP to hold mail until a local mail server calls in to the ISP server to retrieve or "pull" pending messages from the ISP server. If the ISP mail server supports the SMTP ETRN command, you can configure the Domino server to "pull" mail over SMTP. A local Domino server can also use Notes® routing protocols to pull messages from a remote Domino server over a Notes Direct Dialup connection.
Setting up Domino to pull mail from a remote server
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By default, when a local server initiates a connection to a remote server, it uses the connection to push messages to the remote server. The local server does not "pull" pending messages from the remote server. Instead, the local server only receives mail from the remote server when the remote server initiates a connection to route those pending messages. To change this default behavior and have the local server retrieve messages from a remote server during the same session in which it sends messages to the remote server, set up the local server to send a "pull request" to the remote server.
When the local server is configured to send a "pull request," it sends a message to the remote server requesting that the server deliver any messages it has pending for the local server. The remote server receiving the pull request can be any SMTP host; it does not have to be a Domino server. When the remote server receives the "pull request," it checks its mail queues for any messages pending for the initiating server and starts the processing necessary to transfer those messages.
If you are using SMTP routing, you must make sure that ETRN protocol extension has been enabled on the other server (the one receiving the "pull request"), or it will not be able to receive the pull request. Also the remote server must be able to resolve the DNS host name of the initiating server to an IP address to ensure that the messages can be sent. Generally, ETRN requires that the initiating server has a static IP address, which is available in DNS to the server holding the pending messages.
When configuring dialup routing, you can indicate how long the initiating server keeps the line open to allow the remote server to establish a connection. This is useful to prevent the initiating server from hanging up the line before the remote server is able to attempt to transfer any pending mail. The initiating server sends a pull request, then pushes any messages it has for the remote server, and then waits for any messages pending from the remote server.
When sending a pull request, the initiating server can also request messages for other servers, domains, hosts, or any queue name within your organization for which the initiating server is responsible.
The ETRN command
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With ETRN support, a dialup SMTP host can notify an SMTP server holding messages for it when to deliver those messages. ETRN enables servers to use bandwidth resources efficiently, because the dialup host sends and receives mail during the course of a single session.
ETRN stands for Extended Turn and is an SMTP service extension command, defined in RFC 1985. that provides improved security over the SMTP TURN command, originally defined in RFC 821. The TURN command allows hosts involved in a SMTP session to reverse their respective roles, so that, for example, if Server1 is sending an SMTP message to Server2, Server1 can issue the TURN command so that Server2 then becomes the sender and Server1, the receiver.
However, because the TURN command has no mechanism for verifying the identity of the calling host, use of the command poses a security risk. A malicious user who spoofs the identify of a server can falsely appear to belong to a someone else's Internet domain and then use the TURN command to retrieve messages intended for that domain.
The ETRN command plugs this security hole by redefining the sending and receiving roles during the course of the SMTP session. For example, after Server1 issues the ETRN command to Server2, ETRN instructs Server2 to open a new SMTP session with Server1. Because Server2 has to resolve the name of Server1 to an IP number in the DNS, Server2 is more likely to open a new SMTP session with the correct machine.
For Domino to use ETRN to retrieve new mail over a dialup connection, your ISP must support this command. Check with your ISP to verify whether they support this command or not. You can also verify support for the command by establishing a telnet connection to port 25 of the ISP's SMTP server. After the SMTP session starts, type EHLO and press Enter. The response from the ISP's SMTP server indicates whether the server supports ETRN.