Mail server blacklists

Question

Recently I've had a few clients on a server having issues with a blacklist. I've been googling for about an hour and am a little tired of it, but maybe someone here can help me.

What do blacklists take into account when blocking mail? Typically when people talk abount sender reputation, they are referring to an IP address. However a bounce back that I'm getting is specifically stating that they won't accept the mail due to the name servers used by the domain. So do blacklists also block an a per NS basis as well as an IP basis?

Any information on this would be appreciated.

Answer 1

What do blacklists take into account when blocking mail? - whatever they like, they can produce a list based on whatever criteria they feel like and others can either subscribe to it or not based on whatever criteria they feel like using.

I can create an email blacklist right now for people nicknamed "b1ackjosh", and add your mail server IPs to it (if I knew them, i mean) just because I feel like it and others can use it to block or "spam score" email based on whether or not they feel like it. There's not a lot you can do to control this kind of thing, stop people from making these lists, or stop them using the lists for whatever reason they like.

Answer 2

As the others have said blacklists can blacklist you for whatever reason they choose. The better ones do though publish their reasons and will usually tell you why a particular IP address has been blacklisted. With that information you can work with your customers and the blacklist to have the listing removed.

The first thing to do though is to find out why the server is on the blacklist. Look at the headers on your customers' returned mail, they often contain information about which list was used and what the reason is.

A blacklist checking website is also of some use.

Answer 3

Blacklists vary greatly in the criteria by which they add listings (a blacklist, in and of itself, blocks no mail). Mail recipients, also, vary greatly in the criteria by which they block mail.

Answer 4

mxtoolbox.com is a very nice site for black list check.

Answer 5

As Robert said, they can use any criteria they like for blocking but the specifics are many and varied. e.g.

• Someone reports that your system sent out infected emails (often takes just one report)
• Someone reports that your system is sending out spam (normally requires at least a couple of reports)
• Your system is detected as sending out abnormally large amounts of mail in a short time.

At least some blocklist systems run honeyhots and add the sending system to the blacklist whene certain criteria are met. The actual criteria are not always revealed, as knowing what triggers the detection would allow spammers to adjust their techniques accordingly.