Fix email delivery issues for error code 5.1.10 in Office 365

Hi @ShayJacobs,

Welcome to the Microsoft Q&A forum and thank you for your question.

From your description, it appears that you’ve successfully configured two email accounts that can send and receive messages internally within your organization. However, you're encountering error code 5.1.10 when attempting to send emails to external recipients.

To better assist you, could you please share a bit more information?

• Full Error Message:
  • You can find this in the bounce message you received. Look for the error code and description.
  • If you're an admin, you can also check this by going to Exchange Admin Center > Mail Flow > Message Trace and running a trace to view the full error details.
• Direction of the Issue:
  • Does the error occur when you send emails to external users, or when external users try to send emails to you?
• Inbox Rules:
  • Please check if there are any Inbox rules or Forwarding settings that might be affecting email delivery.

Please know that while our initial response might not resolve the issue right away, your input is incredibly valuable. With a bit more detail, we’ll work together to find the best solution for you.

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