We have created a new IP in our email sending solution and are currently undergoing IP warming. However, certain servers are blocking the emails we send.

Here are some examples:

  1. Emails sent to Microsoft's Outlook were blocked and bounced. This was the first time we sent emails to this address.

  2. When we sent emails to our internal email addresses, some recipients received them while others did not. However, our email sending solution marked the sending as successful. Upon inquiry with our internal security team, it was discovered that some recipients were not receiving emails because my IP was automatically registered in an RBL.

Additionally, when we checked our domain on IP reputation verification sites and IP blacklist check sites, there were no listings on any blacklists. (The site were "https://dnschecker.org/ip-blacklist-checker.php" "https://whatismyipaddress.com/blacklist-check#what-is-this-all-about")

Do internet IP blacklist check sites not check all blacklists? Despite not being listed on any blacklists, what could be the reasons why our emails are not being delivered by our own systems or by Microsoft?

Is there a way to determine which blacklists are adopted by specific ISPs such as Microsoft or Google?

Please kindly guide.


1 Answer 1


Short answer is "no, there is no way to determine which blacklists are adopted by specific ISP's such as Microsoft or Google" - in fact it is possible - probable even that they don't rely heavily on blacklists, preferring heuristics.

Getting into the weeds and nuances hinted at by your question.

If email lands up in spam mailboxes, you can sometimes see hints as to why in the headers. (I know my systems add headers as to what Spamassassin thinks).

If you are warming your IP, you may be sending to-large a volume, or your mail may look suspicious to whatever heuristics the mail scanner is using.

If your IP is in an RBL, you may have your work cut out for you. It is likely you are in a bad IP neighbourhood or your IP address is in a block marked as used by end users, and thus not likely to be a legitimate mail server.

If some mail got through and some didn't, it is likely a volume related as well - ie mail servers get suspicious when large volumes of mail are sent from sources they don't recognise.

Not all blacklists are public, and they could be added and removed by third parties, so no, not all blacklist check sites check all blacklists. That said, if you are only on 1 or 2 obscure or custom purpose ones, its unlikely an IP Blacklist is your [primary] problem.

You have not described how you are relaying emails - but if some of your emails are not being delivered locally and are being sent through your mail server, you will need to investigate its logs, and work through this problem - its not something we can advise as each mail server is difference. You may be able to get indications as to why your email is being blocked by other servers based on the message returned to your SMTP server by the remote server when trying to deliver email. Investigating mail queues on your mail server may also provide hints.

You have talked vaguely about "your email sending solution" but you have not provided much information for us to go on or advise.

  • No, the solution to the question you asked is not Salesforce Marketing Cloud. Salesforce Marketing Cloud can not reach into my mail servers and see how they are configured, so has no way to determine what blacklists my servers are configured to use. The best it can do is try and track email delivery based on who opens emails and lets them know, and get information related to bounces from mails sent through their systems.
    – davidgo
    Commented Mar 27 at 0:42
  • (The solution is Salesforce Marketing Cloud.) I'm sorry I delete comment by mistake. I have an additional question: Can you guess that the two examples mentioned in the post are examples that can occur when the Ip reputation is not good? In other words, if the Ip reputation is not good, or if it is registered as a blacklist, (1) it may be recognized that it has not been sent due to a bounce, or (2) it may be impossible to know whether it has actually been received even if it is shipped well on the solution? Commented Mar 27 at 1:14
  • It is easier to answer the opposite - it is not possible to know for sure that an email was not delivered - the receiving mail server may very well take the email and silently drop it. *(it is possible to know with a high degree of probability that an email was opened if you use some kind of tracking in the email and that tracking is triggered - however even that is not fullproof as some mail servers will artificially trigger links and may then decide not to deliver the email)...
    – davidgo
    Commented Mar 27 at 3:09
  • Emails deemed low reputation are unlikely to be bounced, as bouncing them can have the net effect of helping spammers or causing grief to targets. If an email is bounced its generally been deemed somewhat legitimate by the receiving server.
    – davidgo
    Commented Mar 27 at 3:11
  • When sending email to a mail server that you don't control, you can't make any hard statements about whether an email was delivered, and if it wasn't, why not. There are a lot of variables and different servers are configured differently. The best you can do is make informed guesses, and sometimes the receiving mail will hint at its behaviour.
    – davidgo
    Commented Mar 27 at 3:14

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