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POP: mail.yourdomain.com
(substitute your domain in there)
SMTP: mail.yourisp.com
(substitute what you normally use to send mail with your ISP).
About 50%-60% of all mail delivery systems, including AOL, hotmail,
yahoo, Earthlink, etc., cross-check the sending and receiving mail
servers of each mail message with an "anti-spam" system
called ORBS (http://www.orbs.org). ORBS is a database that is
automatically maintained (read: there is no human to call). ORBS
randomly checks mail servers around the world and employs a number
of questionable techniques to gauge whether or not those mail
servers are vulnerable.
If it suspects vulnerability, it
immediately "black-lists" those mail servers.
The problem is that ORBS is an automated system, with zero
notification, and little documentation, guidance, or accountability.
Mail servers can easily get wrongly black-listed, and it takes time
for the ORBS system to self-correct.
So, if we opened up our mail relay
for our customers to be able to use mail.theirdomain.com,
then our mail server would eventually be shut out and no ISP would
accept anyone's mail that was being sent from our server.
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