550 5.7.54 SMTP; Unable to relay recipient in non-accepted domain Error

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550 5.7.54 SMTP; Unable to relay recipient in non-accepted domain Error

Kmcnet 1,116

Hello everyone and thanks for the help in advance. I am developing a C# .Net 9 Windows Application that uses the SmtpClient to send emails. I have an on premises Exchange Server 2019 that I want the application to connect to for sending emails. I want the C# application to connect to the Exchange Server from a remote location to send the emails. I am trying to send an email to a Gmail account from the application and am receiving a 550 5.7.54 SMTP; Unable to relay recipient in non-accepted domain Error. However, the application is able to successfully connect to the Exchange Server and send emails to internal email recipients. So the application appears to be able to connect to the server and send emails, but not to external emails. However, Exchange users ARE ABLE to send emails to this particular Gmail account. I did try to telnet into the server and send an email and received the same error. So I really have no idea where to look next. In the C# application, I am sending Exchange credentials, but have also tried anonymous, with the same results. Any help would be appreciated.

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Answer 1

Vergil-V 2,115 Microsoft External Staff Moderator

Hi @Kmcnet
Thank you for reaching out to Microsoft Learn Q&A!
Based on your description, it seems you're encountering the error: 550 5.7.54 SMTP; Unable to relay recipient in non-accepted domain when attempting to send email from your application to Gmail.

As forum moderators, we don’t have access to a dedicated testing environment to replicate complex, user-specific scenarios. However, our goal is to provide helpful insights based on available resources. From my research, this error may be caused by unauthenticated or insufficiently authorized clients attempting to send mail to external domains.

To help troubleshoot, could you please double-check the configuration of your Receive Connector’s authentication mechanisms?

Please ensure that the Receive Connector on your Exchange server is properly configured to allow relaying for your application.

Additionally, here are some official Microsoft resources related to Receive Connectors that may provide valuable insights and are worth reviewing:

Receive connectors | Microsoft Learn

Allow anonymous relay on Exchange servers | Microsoft Learn

Scenarios for custom Receive connectors in Exchange Server | Microsoft Learn

If I’ve misunderstood any part of your situation, or if you have further questions or updates, please don’t hesitate to reply. Thank you for your patience and for working through this with us.

If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".

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Kmcnet 1,116 Reputation points

2025-07-24T21:32:59.82+00:00

Thank you for the response. I am unfortunately, not an Exchange expert, however, I believe I did previously review the article Allow anonymous relay on Exchange servers and believe I set it up correctly. In the Scoping tab, I entered the IP of the external server that is trying to connect to the Exchange server (it is not a local IP address). In the Network Adapter Bindings, I entered All Available IPV4 with Port 25. So is this correct? Also, since I am able to authenticate using a user name and password, I'm really not sure why I would need and anonymous connector. In testing, the problem is not isolated to Gmail only as any attempt to send to an external email from my application fails. However, no problems sending via my OWA. Possibly DNS?
Kmcnet 1,116 Reputation points

2025-07-27T17:11:13.5233333+00:00

Here is some additional information. Default front end ::-ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff 0.0.0.0-255.255.255.255; Network adapter all available IPV4 and IPV6 on port 587. Client Front End with same settings; Outbound Proxy front end with same subnets and all IPV$ and IPV6 on port 717; and the OutsideRelay with the external IP address and all IPV4 addresses.
Vergil-V 2,115 Reputation points Microsoft External Staff Moderator

2025-07-28T10:12:36.14+00:00

Hi @Kmcnet
Thank you for your follow-up and patience.
I truly appreciate the additional information you’ve shared; it’s been helpful in narrowing down my research.

Based on what you’ve provided, it seems you're on the right track. As the next troubleshooting step, please double-check whether relay permissions have been correctly configured.

This can be done via PowerShell using the following commands:
For a specific user:

Add-ADPermission -Identity "yourC#AppConnector" `

-User "******@domain.com" `

-ExtendedRights "ms-Exch-SMTP-Accept-Any-Recipient"

For Authenticated Users (this includes any user or service that has successfully logged in to your Exchange server):

Add-ADPermission -Identity "yourC#AppConnector" `

-User "NT AUTHORITY\Authenticated Users"`

-ExtendedRights "ms-Exch-SMTP-Accept-Any-Recipient"

As mentioned earlier, due to the limitations of my environment, I can’t guarantee this will fully resolve the issue in your specific case. However, based on the logic outlined in the official documentation: https://learn.microsoft.com/en-us/exchange/mail-flow/connectors/allow-anonymous-relay, adjusting the user parameter should successfully assign relay permissions. This step is crucial to avoid the 550 5.7.54 SMTP error.

Additionally, could you please share a screenshot to confirm that relay permissions have been successfully applied?

This would help us verify that the configuration is working as expected.

You can also try running the following PowerShell command to check the connector settings:
Get-ReceiveConnector "yourC#AppConnector" | Format-List Enabled,TransportRole,Bindings,RemoteIPRanges

Before sharing, kindly ensure that any personal or sensitive information is removed to protect your privacy.

I hope this information helps clarify the situation and brings you closer to a solution.
If you have any further questions or updates, please feel free to reach out, I’m here to help!
Vergil-V 2,115 Reputation points Microsoft External Staff Moderator

2025-07-30T07:17:21.01+00:00

Hi @Kmcnet
I hope you're doing well
I just wanted to kindly check in to see if you've had a chance to try the suggestions I shared regarding double-checking whether the relay permissions have been correctly configured.
Understanding whether our response was helpful is an important step as we continue working toward a solution.
Thank you for your understanding
Kmcnet 1,116 Reputation points

2025-07-31T01:01:23.86+00:00

Thank you for the follow-up. I will be working on this tonight. However, here is my question. The C# application connects to the Exchange Server and can send email at least internally. So once I have authenticated into the server, why would the application need additional permissions?
Kmcnet 1,116 Reputation points

2025-07-31T01:03:21.3266667+00:00

In your example, what would I replace "yourC#AppConnector" with?
Vergil-V 2,115 Reputation points Microsoft External Staff Moderator

2025-08-01T10:30:48.82+00:00

Hi @Kmcnet
From my understanding, your C# application can send internal emails through the on-premises Exchange server because the server already recognizes the recipients and can route the messages directly.
However, sending emails to external domains requires relay permissions, as those messages go beyond the protected environment of the Exchange server.
Regarding what should replace "yourC#appConnector", you can use the receive connector that you previously set up, as mentioned in your earlier response.
Vergil-V 2,115 Reputation points Microsoft External Staff Moderator

2025-08-04T08:39:33.81+00:00

Hello @Kmcnet
I’m reaching out to follow up, as it’s been a little while since we last discussed granting relay permission to send emails externally. We’d really appreciate any updates you can provide, especially whether the information we shared was helpful. Your feedback is valuable as we continue improving our support.
Thanks again for your time, and we look forward to hearing from you.
Kmcnet 1,116 Reputation points

2025-08-13T00:56:32.7866667+00:00

First of all, thank you for your followup. I did try the powershell commands, but receive errors and I am not sure how to trouble shoot because I am not exactly sure what I am doing. It appears this is trying to add an active directory entry, but I'm not sure why this would be needed since I can authenticate into the server and send emails internally.

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550 5.7.54 SMTP; Unable to relay recipient in non-accepted domain Error

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