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Emails to one domain are bouncing back from a different domain

Anonymous
2021-04-20T15:42:30+00:00

People from my organization are trying to send emails to people in another organization, but we keep getting email bounce messages back from a third domain that doesn't seem to pertain to either of us.

Is this a problem with the organizations configuration that we are sending emails to, or is it possible there's something wrong within our Exchange 365 hybrid configuration or with Microsoft's configuration? If so where would I start looking?

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Anonymous
2021-04-26T01:18:47+00:00

Hello Seth,

Since I checked the NDR you provided last time, the NDR not generated by Microsoft 365 Exchange Online, and also according to the problem you mentioned above, as far as I know the problem is more likely related to the on-premises Exchange server side of misconfiguration. In this case first please make sure that your local Exchange server firewall is not the issue for the mail flow now, it should allow all the Office 365 URLs and IP address ranges >> Exchange Online.  And also make sure your Exchange server domain SSL certificate is valid for Hybrid.

Then if they are OK, in this case it is recommended that please re-run the latest version of  Hybrid Configuration Wizard (HCW >> https://aka.ms/HybridWizard) and then see if it could help you update and fix the mail flow connector issue in this round.  And make sure your Exchange server Outbound Connector can send emails to internet, thanks.

On another hand, if the issue persists, in this case as the local Exchange server Connectors in your scenario is not working, however since we are mainly focusing on Exchange Online for Business Support, we are not dedicated for Exchange On-premsies server related, if you still experience this issue, it is recommended that please post a new thread at Microosft Q&A, the dedicated support engineer there are Exchange servers related, and they would give you more professional assistance regarding your Exchange server connectors.  By the way, if you need any other helps from Exchange Online, please also feel free to let me know, and I am willing to help you do more research, thanks.

Your understanding and patience will be highly appreciated.

Best Regards,

Oliver

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  1. Anonymous
    2021-04-23T17:45:51+00:00

    Oliver,

    We ended up having even bigger issues, so I'm not sure if our ultimate fix is related to this or not.

    Ultimately our biggest problem ended up being that our exchange server wasn't getting regular updates due to a misconfiguration in Windows Update. It was so far out of date that it likely caused our connector to stop working. The odd behavior leading up to it not working altogether may or may not have been related as we had multiple people looking into multiple possible reasons, and we do not have confirmation from everyone as to what was found. This includes people outside our organization on the recipients end, and Microsoft 365 support personnel.

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  2. Anonymous
    2021-04-22T09:19:22+00:00

    Hello Seth,

    Thanks for your reply in the PM with patience.

    I've checked the whole NDR and the sender's email address with recipient's.  and also you mentioned when you sent from a personal email account to the recipient, it is OK, this means the recipient side should be OK in this round, and the issue is more likely related on sender's side.  

    From the NDR, I checked the recipient points MX of his domain to Exchange Online Protection (EOP), but your NDR not generated from EOP but instead of " CoR-EntMExSvr01.rex xxxx.us" ( it looks very strange).  If you have Exchange Hybrid with Centralized mail enabled, all the external outbound emails will be routed from Exchange on-premises server, and the NDR seems not generated from your Exchange server and not the recipient EOP.

    In this case, if your outbound emails routed via on-premises Exchange server, please double check what connectors are used to send these emails from your server out, and if you have a lot of outbound connectors, please make sure you have set up the correct one to route emails.  Also may I know if you have any third-party anti-spam server/services in front of your outbound server?

    Please feel free to share with me if you got any further updates, thanks.

    Best Regards,

    Oliver

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  3. Anonymous
    2021-04-21T16:47:47+00:00

    Thanks Oliver,

    To answer your questions:

    1. We have tested both on-premises exchange account, and office 365 accounts. They both behave the same.
    2. We could send emails before to and from the same emails. Nothing should have changed with our connector during this time.
    3. I'll send you a PM back.

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  4. Anonymous
    2021-04-21T00:32:58+00:00

    Hello Seth,

    According to your description, the email bounced back from a different domain than the email address (domain) your users sent.  Generally this scenario would happen when your users sent to an email alias of other users in another organization.  However, I noticed you mentioned the None-Delivery Report message bounced back from a domain seems not belongs to both of you.

    In this case to further check if the issue is caused by your Exchange Hybrid organization Configuration or the recipient side limitation. First you could try to test if your outbound mail flow is working, you could try to send emails out to some external emails (like Outlook.com) and then see if you still facing the email bounced back. If it is working, this means the problem is more likely related to the recipient side. And to further check the issue, I'd like to collect

    1. If your sender mailbox is Microsoft 365 Exchange Online mailbox or a local Exchange server mailbox?
    2. If you can send emails to the people in another organization before, and if you modified any settings in your Exchange Hybrid like connectors?
    3. Please provide the full Non-Delivery Report message to me for further checking.

    To protect your privacy, I will send you a Private Message (PM), please share it with me there, thanks.

    Best Regards,

    Oliver

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