551 5.1.6 Permanent Failure

Mailbox has moved, no forwarding address

The mailbox previously existed at this address but has since moved (often due to a domain migration or account restructuring) without a forwarding rule in place.

Common causes

Company migrated email providers or domains.
User account was renamed or restructured without a forward.

What to do

Suppress the address as a hard bounce. If you have another contact point for this recipient (a different email, a company-wide alias), consider following up through that channel instead.

FAQ

Is this common?

Relatively rare compared to 5.1.1, but it does occur during company email migrations or rebrands, where an entire domain's mailboxes change without automatic forwarding configured.

Other permanent failure codes

550 5.1.1

User unknown / mailbox does not exist

550 5.7.1

Message rejected due to policy (spam/content)

552 5.2.2

Mailbox full (permanent quota failure)

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