If using Microsoft Exchange, you must have version 5.5 or greater. The way ETRN works is pretty simple. Whenever your mail server is dialed into the Internet, any mail being sent to your domain will come to you directly. During this time, your mail server will connect to ours and issue an ETRN command, which will cause our mail server to send any queued mail for your domain. Whenever your mail server is not dialed in, one of our servers will be queuing the mail for your domain, and the cycle repeats itself.
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