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Deliverability · Practical guide

SMTP 550 user unknown: stop retrying an invalid destination

Confirm an explicit invalid-recipient response, suppress the failed address and investigate the source of the contact data.

Reviewed · Examples are illustrative

Who this helps: Operators diagnosing authentication, receiving-policy and delivery failures.

Define the decision

The full response matters: 550 can describe several permanent failures. When the receiver explicitly says the mailbox does not exist, repeating the same address is unlikely to repair the problem.

Work through the procedure

  1. Confirm the recipient address and full rejection text.
  2. Apply the appropriate suppression through the contact workflow.
  3. Check the source record for a typo, stale employee or malformed import.
  4. Review other contacts from the same source before enrolling another batch.

Worked example

The following is a synthetic example for this procedure, not a customer result or performance benchmark.

Recipient: [email protected]
Response: 550, mailbox does not exist
Action: preserve the original failure and suppress this address
Data review: source list was collected before the employee changed roles

Read the result

The finding may be a data-quality issue affecting more than one record. Correcting a typo should produce a reviewed contact identity, not silently erase the history of the failed address.

Check before moving on

  1. Verify the address is ineligible for later campaign steps.
  2. Keep opt-out suppression separate from address correction.

Limits and next action

Do not guess nearby addresses and retry them automatically. Use a verified source and review the contact’s relevance before any new enrollment.

Source: RFC 3463: enhanced mail status codes

Source references

Worked examples are illustrative. Editorial procedures are suggested methods, not measured performance claims or promises of additional product features.

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