smtp bounce code
550 5.1.10Recipient not found by SMTP address lookup
what to do
Suppress the address
Microsoft 365's version of user unknown. The address does not exist in the tenant.
What the server is telling you
Exchange Online could not resolve the address to a mailbox, group or contact. Unlike 5.4.1, this one names the lookup as the failure, so it is reliable evidence the address is not real.
Next steps
- →Suppress the address.
- →Check for a typo, especially in the local part.
Commonly returned by: Microsoft 365 / Exchange Online
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