What is the extended turn (ETRN) mail pickup on Exchange Server 5.5, and how does it work?
ETRN is a command that an SMTP server uses to start delivery of queued mail. In ordinary SMTP conversations, a server sending mail addressed to a recipient on another server will open an SMTP connection directly to the recipient's server to deliver the mail. However, the sending server delivers the mail only if that server can directly contact the receiving server; otherwise, the sending server queues the mail. Because SMTP is a store-and-forward protocol, any host can accept mail for another computer and deliver the mail in the future. For example, if my SMTP server goes down, I have a secondary DNS MX record pointing to another host that can accept my mail, queue it, and deliver it to me when my SMTP server comes back up. As an alternative, your server can connect to the Internet, connect to the server that's holding your mail, and send an ETRN command to the server to begin forwarding your mail to your server. This process, known as dequeuing, is fairly simple to set up. For complete details, see Simpler-Webb's excellent FAQ at http://www.swinc.com/resource/exch_dq.htm.