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cPanel rejecting Exchange emails (large number of recipients)

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5 comments

  • cPRex Jurassic Moderator

    Hey there!  Yes, this is expected behavior on the system.  In cPanel >> Exim Configuration Manager, we have the following options related to this error you're seeing:

    • Dictionary attack protection
    • Ratelimit suspicious SMTP servers
    • Ratelimit incoming connections with only failed recipients

    The best solution would be to ensure that all the email-cpanel01 through email-cpanel100 addresses are all working properly.  If the failures quit happening, you'll quit receiving that bounce error.

    You can also turn those settings off if you don't think it will cause any issues with your configuration.

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  • unifique

    Hello cPRex,

    Thank you for your attention to my case. When you say "to ensure that all the email-cpanel01 through email-cpanel100 addresses are all working properly" do you believe it could be something related to Exchange? I'm sure the accounts that are working on the cPanel infrastructure are working correctly, as there were no problems until some boxes were changed from the client to Outlook.

    Regarding deactivating the resources you mentioned, I believe it is not the best option, as it is a shared environment and not dedicated to this client.

    Can you tell me if using separate email boxes on two providers (one of them being cPanel) is a good practice or the best way to do this? The cPanel's email routing is set to "Local", perhaps this could impact this situation?

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  • cPRex Jurassic Moderator

    *Something* is telling Exchange that some of those email addresses are failing.  I'm not sure which addresses based on the log data here, but cPanel can't deliver to some of the addresses for some reason.

    No, I think you have things configured properly.  Personally, I probably wouldn't try that type of split delivery and I've put everything on one side or the other, but that's personal preference.

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  • unifique

    Hello,

    Just an update on my case. After doing some research and testing in my environment, I was able to solve my problem. I believe that due to the high number of senders, a large number of relay attempts were generated over a short period of time from Exchange to cPanel. I believe that Exim interpreted this as a possible attack and blocked the receipt of these messages. I was able to get around the situation by including the Exchange Online IPs in these Exim lists: 

    Exim Configuration Managar > Access Lists > "Trusted SMTP IP addresses" and "Sender verification bypass IP addresses".

    I got the IPs through the link: https://learn.microsoft.com/pt-br/microsoft-365/enterprise/urls-and-ip-address-ranges?view=o365-worldwide

    After I made this adjustment, I had no more problems.

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  • cPRex Jurassic Moderator

    Nice!  I'm glad you found a good solution for that issue!

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