Mailbox not receiving external email

Question

I can send and receive internal email successfully, but external senders receive an NDR when sending to the user. Other users in the same domain are unaffected. What Exchange Online components would you investigate?

Answer 1

Hi Mia Collins

Since internal email works correctly and other users in the same domain can receive external messages, troubleshooting can initially focus on the affected mailbox and recipient-specific mail flow rather than the domain as a whole.

The NDR is the best starting point. Check the enhanced status code, such as 5.1.1, 5.4.x, or 5.7.x, along with the Diagnostic information for administrators section. These details can indicate where the message was rejected and help narrow down the cause. Microsoft provides information about common Exchange Online NDR codes and their possible causes here: Email nondelivery reports (NDRs) and SMTP errors in Exchange Online - Exchange | Microsoft Learn

Some areas to check in Exchange Online include:

Recipient configuration: Check the affected mailbox in the Exchange admin center and verify that the primary SMTP address and aliases are correct, particularly the exact address being used by the external sender.

Message trace: Run a message trace for one of the affected external messages. This can help determine whether the message reached Exchange Online and where processing or delivery stopped.

Message trace in the new EAC in Exchange Online | Microsoft Learn

Mail flow rules and recipient restrictions: Check for transport rules, delivery restrictions, or other mail-flow settings that could apply specifically to this recipient or email address.

Mail routing and connectors: If the organization uses hybrid mail flow, a third-party mail gateway, or custom connectors, check whether this recipient is being routed differently from other users. Connectors can customize mail flow between Microsoft 365 and on-premises servers, partners, or other mail systems.

Configure mail flow using connectors in Exchange Online | Microsoft Learn

Accepted domain configuration: This is less likely to be the primary cause when other users in the same domain receive external mail successfully, but it can still be reviewed if the previous checks do not identify the issue. Exchange Online uses accepted-domain configuration to determine how messages for recipients in a domain are handled.

Manage accepted domains in Exchange Online | Microsoft Learn

If possible, please share the NDR error code and the diagnostic information after removing email addresses, domain names, IP addresses, server names, and any other identifying information. That information can help narrow down the specific component involved. (Before sharing the NDR here, please remove any email addresses, IP addresses, server names, or other information that could identify you or your organization)

If you do not have administrator permissions for your organization, some of these checks, such as message trace, mail flow rules, connectors, and recipient configuration, may not be available to you. In that case, please contact your organization’s Microsoft 365 or Exchange administrator and provide them with the NDR, including the error code and diagnostic information, so they can investigate the mail flow from the administrative side.

I hope this information is helpful, and please feel free to let me know if you have any further questions.