Today I one of our customers forwarded to me a mail from the ISP of one of her mail recipients that says they're blocking my customer's email because our webserver IP is listed in some RBLs.

Of course the latter, being a webserver, has nothing to do with this email journey through the 'net and unfortunately there isn't any DSN / bounce to look at: all we got is an email from someone working at the recipient's ISP saying "mail gets blocket because $ip_of_webserver is in $someRBL but I don't know why our system are checking that ip in the RBLs".

Detailed info:

  • the blocked mails have sender domain1.com and recipient domain2.com
  • the blocked mails started on the user's MS Outlook, which sent them to mail.ispdomain.com, which sent them to smtpout.ispdomain.com, which sends them to the MX of domain2.com
  • the MX of domain2.com does accept the email from our smtpout and responds 250 Requested mail action okay, completed - it probably filters the email in a later phase
  • domain1.com has MX records mx.ispdomain.com
  • mail.ispdomain.com, mx.ispdomain.com, smtpout.ispdomain.com are not listed in any RBL
  • webserver.ispdomain.com is listed in SORBS-WEB and SORBS-SPAM
  • only that specific IP is listed and not a whole range or subnet
  • webserver.ispdomain.com relays all of its locally-generated mails through smtpout.ispdomain.com
  • the blocked email did not come from or pass through the webserver
  • all the servers have correctly configured PTR records on their IP addresses

This is still an open issue and it will be interesting to see how a webserver that relays all of its mails to another system got listed in an RBL, but that's another story.

I am not asking "why doesn't my mail work" or how to de-list my system. Been there, done that.

I see that the only connection between an email and a webserver IP can be the domain name of the mail's recipient (possibly present in the mail text too, eg. in the signature). I'm guessing some antispam system took that domain name (from recipient address or from mail body), resolved its A dns record, and looked that up in RBLs.

I am asking: is there some widespread antispam software / technique that behaves like that? And is that actually an effective measure against spam?

closed as not a real question by Michael Hampton, Magellan, MadHatter, Tom O'Connor, mdpc Nov 8 '12 at 17:35

It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, visit the help center. If this question can be reworded to fit the rules in the help center, please edit the question.

  • Thanks to @MadHatter for editing my subject which was a little too twisted on itself :) please forgive me but I learned English at school. :| – Luke404 Nov 8 '12 at 7:43
  • Luke, don't worry about it, it happens a lot around here. Thanks for taking it so gracefully - and I can assure you that your English is vastly better than my Italian. That said, this question may get closed soon. For my money, it could be hugely improved by including the whole text of the bounce email, so that we can examine the issue from beginning to end - because what you ask now isn't really a question that can be answered. – MadHatter Nov 8 '12 at 7:45
  • @MadHatter, I just had my coffee and started my workday, and I'm actually editing my question right now to improve it based on feedback from current answers. Unfortunately there isn't any bounce to write about. – Luke404 Nov 8 '12 at 7:47
  • Then I submit that there isn't really a question that you can answer, either. If I were brought an issue like this in my professional capacity, I'd refuse to touch it unless I could get hold of a transcript of an actual bounce. Otherwise, there's just so much that can be lost when you listen to a user's report, of another user's report, of a technical problem; you end up chasing the wrong issue nine times out of ten. And now you mention it, an espresso would be a lovely idea... let me make myself one! – MadHatter Nov 8 '12 at 7:49
  • I'm trying to phrase it better, but the matter of the fact is that I'd like to ask about antispam techniques in general and if there is any that could behave like that, I'm not asking for help to get this specific message through. – Luke404 Nov 8 '12 at 8:01

Hmm, interesting question. First, a clarification.

Today I one of our customers forwarded to me a mail from the ISP of one of her mail recipients that says they're blocking my customer's email because our webserver IP is listed in some RBLs.

Your customer tried to send an email and the mail was bounced back to her by the recipients ISP, correct?

If that is the case, the bounce mail should have the specific SORBS code for why it was bounced. I would look that up here: http://www.sorbs.net/general/using.shtml This should tell you which list it is blocked in.

From their site, the error codes are:


Their description of each list (at the link above) will tell you if you are being blocked because of a bad block of IPs, a single IP, a domain which has requested not to be listed in their RBL, SPF violations etc.

From there, it should be easier to track down the specific cause.

Update: If you can't get the actual bounce message, you might be able to look up the ip of the sending mx server here: http://www.sorbs.net/lookup.shtml To get the details and save the guess work.

Without a bounce message from the recipients ISP, it is harder to track down which of SORBS lists you are running afoul of. It is also strange that the recipients ISP is sending out hand generated emails rather than sending machine generated more detailed ones, but there is nothing we can do about that :)

Normally, I would try doing a manual SMTP connection to the recipients ISP, and see if you can get a better message from it. However it looks like you have already confirmed that their mail server is accepting the message (at least at this stage).

Guessing blind, the only things I could think of would be:

  1. An SPF problem (eg the sending mx server not authorized to send mail for the domain via DNS SPF records)
  2. The following item on SORBS might somehow be implemented to block all email originating from a domain name associated with spam:

escalations.dnsbl.sorbs.net - This zone contains netblocks of spam supporting service providers, including those who provide websites, DNS or drop boxes for a spammer. Spam supporters are added on a 'third strike and you are out' basis, where the third spam will cause the supporter to be added to the list.

Especially since domain1.com has MX records mx.ispdomain.com, SORBS might link the two. Ie. it groups all hosts for a domain together based on their A and MX records.

BTW, it might also be a good idea to fix the problem with the mail server and get it de-listed also. It looks like the SORBS list checks for open relays, broken scripts and such that you would want to close no matter what the case.

  • The involved RBLs are already cited in the original question and are SORBS-WEB and SORBS-SPAM. – Luke404 Nov 8 '12 at 7:48
  • I clarified the question and explained there is no bounce to look at. I'm trying to make it clear that my question is actually about what antispam techniques could behave like that (because I don't know any that would) and not about solving the specific issue of this specific mail. – Luke404 Nov 8 '12 at 7:59
  • @Luke404 just updated the post based on your update. – jpgeek Nov 8 '12 at 8:33

It could be that your subnet was blocked and not just your IP address, also the ISP isn't going to no much about why you are blocked -- the best thing to do is go to http://www.mxtoolbox.com/blacklists.aspx and check what black lists are blocking you and get in contact with them.

  • Thanks for trying, but the original question already clarified that only one specific IP address was listed. – Luke404 Nov 8 '12 at 7:58
  • @Luke404, Read right over that without seeing it. – Winter Faulk Nov 8 '12 at 16:10

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