Emails from external domains to mailboxes with forwarding enabled to External accounts are not getting delivered to External accounts

Mani 446 Reputation points
2025-04-03T16:37:27.9133333+00:00

Hello All,

We are facing an issue with Mail flow,

When an user send email to external domain it's working

Email from internal to internal working

Internal to external working

External to internal working

But the Mails from External domains to mailboxes with forwarding enabled to external accounts are not getting forwarded to external accounts (Actual mailbox is receiving that mail but not forwarding)

We enabled forwarding from Exchange admin center

Please assist what might be the cause for this (This stated few days back)

Error Details
Error Details
Error:550 5.7.367 Remote server returned not permitted to relay -> 550 5.7.1 Relaying DeniedError:_550 5.7.367 Remote server returned not permitted to relay -> 550 5.7.1 Relaying Denied_Message rejected by:mx0a-00256401.pphosted.com
Exchange Online
Exchange Online
A Microsoft email and calendaring hosted service.
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3 answers

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  1. Amit Singh 5,306 Reputation points
    2025-04-04T03:54:28.75+00:00

    Hi Mani,

    This issue likely started because of changes in Microsoft 365 security policies.

    By default, Microsoft blocks automatic forwarding to external domains to prevent data leaks or spam abuse.

    You can allow automatic external forwarding by changing a setting in Exchange Online:

    1. Go to Microsoft 365 Security & Compliance Center: https://security.microsoft.com
    2. Navigate to: Policies & rules > Threat policies > Anti-spam policies
    3. Open the Default (or custom) anti-spam policy.
    4. Under Outbound spam filter policy, find the setting: "Automatic forwarding".
    5. Change it from "Automatic - System Controlled" or "Off" to: "On - Allow automatic forwarding".
    6. Save the policy.

    You can check this - https://learn.microsoft.com/en-us/defender-office-365/outbound-spam-policies-external-email-forwarding

    Let me know if you have any query.


  2. Anonymous
    2025-04-04T05:56:35.9866667+00:00

    Hi,@Mani

    Thanks for posting your question in the Microsoft Q&A forum.

    Based on your description, I'd like to confirm a few things with you:

    1.Is your email system sending and receiving properly?

    2.Regarding mail forwarding, did you set up mail flow rule?

    3.Can you provide the full NDR?

    4.According to this sentence “Actual mailbox is receiving that mail but not forwarding”, does it mean that the mailbox responsible for forwarding is not receiving the mail?

    Suggest you use mail flow to see exactly where the email is stuck.


  3. Andy David - MVP 157.8K Reputation points MVP Volunteer Moderator
    2025-04-04T13:49:56.9533333+00:00

    That error message means the recipient is blocking it. Not much you can do there.

    Error:550 5.7.367 Remote server returned not permitted to relay -> 550 5.7.1 Relaying DeniedError:_550 5.7.367 Remote server returned not permitted to relay -> 550 5.7.1 Relaying Denied_Message rejected by:mx0a-00256401.pphosted.com

    AutoForwarding to an external recipient is prone to failures regardless because the message will typically fail SPF/DMARC

    Exchange Online uses SRS to address the problem:

    https://learn.microsoft.com/en-us/exchange/reference/sender-rewriting-scheme

    DKIM can help here:

    https://learn.microsoft.com/en-us/defender-office-365/email-authentication-about#how-spf-dkim-and-dmarc-work-together-to-authenticate-email-message-senders

    https://postmarkapp.com/blog/forwarding-emails-dmarc-failure


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