Why was my SMTP connection rejected?
Open proxies and open relays are a very common source of spam. Yahoo! uses a variety of techniques to detect these, and Yahoo! does not accept SMTP connections from either open relays or open proxies. Your connection may be rejected because the email server that you use for sending email is probably an "open relay" or "open proxy." These types of email servers can easily be tricked into sending large amounts of unsolicited email.
Many email services refuse mail from such servers, until the server has been fixed to stop relaying spam to innocent users.
To correct this problem, you should contact the system administrator of your mail server (usually "firstname.lastname@example.org") and forward them a copy of the bounced email you received. Otherwise, you should call technical support and alert them of the problem.
You can check to see if your mail server is listed on several of the public open relay DNSBLs. Please note that if your server is not listed at this link, it may still be an open proxy or open relay. Abuse.net provides a web-based open relay test. Instructions to close open relays are readily available on the Internet.
To determine whether an IP address is an open relay or open proxy, a series of tests need to be performed on that IP address. Yahoo! actively performs tests for open relays and open proxies but only after that IP address has been identified as a source of significant suspicious inbound traffic to the Yahoo! network. In other words Yahoo! only makes reactive tests triggered on inbound traffic from that IP address, and pro-active tests are never performed by Yahoo!.
Yahoo! strives to ensure that any testing performed as a consequence of the inbound traffic triggers have the least impact possible on the tested address.
Once an IP address has been tested, no further tests are performed for at least three days regardless of the outcome of the tests.
Actions based on test results
If an IP address is determined to be an open relay or open proxy, Yahoo! reserves the right to reject all SMTP traffic from that IP address for a minimum period of 60 days.
Source address of tests
All tests run by Yahoo! are run on readily identifiable servers that have forward and reverse DNS entries ending in yahoo.com. Any email addresses and servers used as part of the testing likewise are always identified with yahoo.com DNS entries or yahoo.com email addresses.
How to remove your server
You should check (and secure) your server for open relays (such as http://www.abuse.net/relay.html) and open proxies (such as http://www.unicom.com/sw/pxytest/ -- these may require installing software on servers outside your network for proper testing). After you are reasonably sure your network is secure, please submit your IP addresses to Yahoo! for retesting.