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We send legitimate e-mails to our opted-in customers in bulk. Recently we started having issues with our IP reputation, which we are working on to improve. We are digging into the issue to find the root cause and would like to know from where does various RBLs and DNSBLs collect data from? For example, I read from SORBS FAQ that they have a set of 'feeder servers' from which data is collected.

Are these feeder servers the mailservers of various reputed email service providers like Gmail or Hotmail?

From SORBSs' website:

6: What and where are the feeder servers..?

The 'feeder' servers are a selection of servers that send addresses of selected the incoming connections to the SORBS servers for testing. The list of feeder servers will not be disclosed, by request of those server administrators.

I am not looking for an answer to the things we should do to keep us away from the list, but trying to find out how these RBLs/DNSBLs collect the info.

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    There are too many ways that these RBL sites collect data to detail here. There are honey-pots, spam filters, pattern recognition, manual reporting, etc. If you are trying to avoid triggering a system by reverse engineering what the RBLs do, that would be useless. Make sure your list is comprised of real users, valid e-mail addresses, has a clear opt-out link, and any e-mail sent is properly formatted with honest header information. It may help to use a reputable e-mail service instead of doing it yourself. Keep in mind that not all e-mail services are equal or good. – closetnoc Nov 11 '16 at 16:39
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    Also keep in mind that almost any sites IP address will end-up on a list for one reason or another. Some RBLs are seriously flawed or engage in practices that never really made sense. So trying to avoid being on a RBL is almost futile. Just do good work and keep your head down. Avoid risk as much as possible. – closetnoc Nov 11 '16 at 16:42
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There are a variety of data sources used by RBLS's to identify potential spam sources...

Honeypots - In these instances fake email addresses are created and posted in order to receive spam emails. When spam emails are received then the sender is added to blacklists.

Spam Filters - Many spam filters provide regular feeds to blacklist providers based on automated spam detection algorithms. When a spam email is detected by the spam filter not only is it flagged as spam in the email client but the details of the email sender and email server are also sent through to various blacklists.

Manual Reports - Many manual spam report buttons result in spam reports being sent through to blacklist service providers as well as flagging the email as spam. This works very much like the spam filters process but rather than the spam being flagged automatically a user has seen the email and has manually flagged it as spam using the "this is spam" button.

Reputable Email Providers - Many free and commercial email providers such as Live Mail, Gmail, Yahoo Mail, etc, have their own in house spam filters and feed spam data through to blacklists.

There are a large number of other methods that blacklist service providers have for detecting spam on the internet and these methods are often in house proprietary methods which do not get released. The reason why it is so hard to find out how emails are detected as spam and reported is that providing that sort of information would make it easier for spammers to implement blackhat techniques to bypass and avoid detection and reporting on blacklists.

As for what the SORBS feeder servers are basically "feeder server" is a generic term used to describe the source servers of data which are often either a receiving email server or a spam filtering services main database.

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