550 5.1.10: Recipient not found

Microsoft 365SMTP

The error usually looks like this:

550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient not found by SMTP address lookup

What it means

The recipient’s email address doesn’t exist in the destination Microsoft 365 organization.

Common causes

  • The address is misspelled, or the mailbox has been deleted or renamed.

How to fix it

  1. Check the recipient’s address, or contact the recipient another way to confirm it.

Tools to help diagnose it

Email header analyzer

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