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Tutorial How do I check if my mail server is blacklisted?

Insight on how to check if your mail server is blacklisted.

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    Overview

    There are many instances in which external mail servers may block mail from your server, resulting in outgoing mail attempts being rejected with a bounce-back error.

    Typically, this issue is caused by your server's sending IP or domain being listed in an existing RBL (Realtime Blackhole List). Some providers may also block emails based on the domain or IP's 'sending reputation' being noted as unknown or in poor standing.

    This guide will provide insight on how to check if your server is blacklisted, where to check sending reputation, as well as what to do when a larger provider (such as Gmail) has blocked emails from your server.

    The information here is meant to assist in removing your server from a blacklist after the reason for blacklisting has been addressed. Attempting to remove a server from a blacklist while malicious activity is still present is not recommended.

    For a general idea of how to prevent email abuse on your server, I would suggest reading our documentation on the topic.

    How do I know if mail from my server is blacklisted?

    The following resources allow you to check if your IP, hostname, or sending domain are registered in any existing blacklists:

    MultiRBL.valli.org
    MxToolbox
    rbls.org

    The following resources are limited to checking IPs, but may also prove useful when confirming if your server's sending IP is blacklisted:

    Abuseat CBL
    WhatIsMyIPAddress.com - IP Address Blacklist Check

    Please note, some existing RBLs may block your server's IP without reason, which means that some listings can be ignored as false-positive. To provide a few example scenarios:
    • The country associated with your server's IP may be blocked in a country-based blacklist.
    • Inversely, some blacklists block IPs from all countries except for a single country (i.e., IPs outside of Denmark are blacklisted automatically).
    • The blacklist may arbitrarily block your server's IP per a provider-specific metric.
    For reference, here are the most common RBL providers which are used:
    How do I check my sender reputation?

    Here are a few resources that provide current data for a server's sending reputation:

    Sender Score (Return Path)
    Cisco Talos - IP and Domain Reputation Center
    BarracudaCentral
    ReputationAuthority (WatchGuard Technologies)

    Providers can use IP and domain reputation to identify the history of suspicious email activity regardless of either being present in a blacklist. This information can be useful for mitigating against 'repeat offenders' or even potentially malicious sources that are yet to be blacklisted.

    Unlike blacklisting, reputation is something that is built upon through the sending trends of a server, somewhat like a 'credit score.' In some cases, reversing negative sender reputation can prove more difficult than being removed from a blacklist, so paying attention to this reputation is crucial.

    [Provider] is blocking my email, how do I remove this block?

    Major email providers will often use individual blacklisting to mitigate against email that has been reported as suspicious, which can provide a further challenge to having your emails deliver successfully once more.

    Below is a list of the most common blacklisting errors found with large providers, and what can be done to address each of them.

    Please note, cPanel, L.L.C. cannot assist you in this blacklist removal process:

    Gmail

    Example error response:
    What to do next?

    As per the error message, Gmail suggests reviewing their documentaiton first to determine why your outgoing mail attempt was blocked.

    Who do I contact to remove this block?

    You can report a delivery issue between your domain and Gmail here.
    Microsoft (Hotmail, Outlook, Live)

    Example error response:
    What to do next?

    The error response from Microsoft suggests to contact your hosting provider, as this error may indicate a network-wide block. They also suggest referring your provider to their troubleshooting page.

    Who do I contact to remove this block?

    You may wish to first contact your hosting provider, depending on the content of the error response.

    Regardless, Microsoft allows you to report delivery issues to their mail services here.
    Office365

    Example error response:
    What to do next?

    The error response may vary depending on the reason for being blocked, so it is suggested to review Microsoft's error response documentation to find specific instrucitons for resolving the issue.

    Who do I contact to remove this block?

    Microsoft provides instructions on how to be delisted from their 'blocked senders' list here.
    Yahoo

    Example error response:
    What to do next?

    Yahoo suggests reviewing their PostMaster documentation for additional insight on why you have been blocked or deferred.

    Who do I contact to remove this block?

    Yahoo suggests contacting their PostMaster team directly to resolve most issues. Alternatively, you can apply as for a 'new sender' status on Yahoo's servers here.
    AOL

    Example error response:
    What to do next?

    The error response may vary depending on the reason for being blocked, so it is suggested to review AOL's error response documentation to find specific instrucitons for resolving the issue.

    Who do I contact to remove this block?

    You can request to have any existing AOL blocks removed here.

    GMX

    Example error response:
    What to do next?

    The error response may vary depending on the reason for being blocked, so it is suggested to review GMX's error response documentation to find specific instrucitons for resolving the issue.

    Who do I contact to remove this block?

    You can report a delivery issue between your domain and GMX here.

    Other Providers

    Any providers not mentioned here (i.e., ProtonMail, Yandex, Zoho) do not provide explicit blacklist removal instructions. You will need to contact their support teams for further insight.

    Conclusion

    In this tutorial, you have learned how to check for server domains and IPs in popular blacklists, as well as a few methods of analyzing sender reputation. You have also learned how to handle provider-specific blacklisting issues with their available resources.

    If you would like to know more about ensuring email deliverability, you should check out our resources on preventing email abuse and inbox delivery:

    How to Prevent Email Abuse
    How to Keep your Email Out of the Spam Folder
    How to Prevent Spam with Mail Limiting Features
    Mario-cPanel likes this.
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