A nondelivery report containing the error code 521 indicates that a specified mailing system is blocked because it has displayed behavior consistent with spamming or other forms of abuse of network resources. This behavior has been noted by our system, by other reputable monitors of network activity, or by both. If you think your IP has been misidentified as a source of spamming or other questionable behavior, we recommend taking the following steps:

1. Visit the websites of the third-party sources:
     http://www.spamhaus.org/index.lasso
     http://ipremoval.sms.symantec.com/

If you are on either of these lists, follow the directions provided there for requesting removal.

AND . . .

2. Visit the page for requesting removal from the list we create:
     http://www.att.net/general-info/bls_info/block_admin.html

Please note that the processes for requesting removal are intended to be completed by system administrators, not individuals. Processing a request requires information that only administrators will know. If you are an individual user whose messages have been blocked, please send a message to your administrator pointing him to this page (http://www.att.net/general-info/bls_info/521.html). In most cases, you can reach an administrator through the Postmaster address for the system you are using (postmaster@[your_e-mail_system.net]).

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