SMTP 550 errors: distinguish recipient failures from policy blocks
A 550 response indicates a failed SMTP action, but the number alone is not enough to identify the fix. Read the full diagnostic text and the stage at which it occurred before deleting a contact or changing your sender setup.
Open the relevant free tool →The text after 550 matters
Capture the full response
Save the extended status code, message text, recipient, timestamp and sending service. A short dashboard label can hide the distinction between an unknown recipient and a policy restriction. If you only have “550,” open the original delivery report or your provider’s detailed event view. Redact private content before sharing the report, while retaining the technical evidence.
Identify the failing scope
If one old address fails while other recipients at the domain work, investigate that address. If many unrelated recipients fail from the same sending stream, inspect shared sender settings. If an entire destination organization fails, compare its routing and policies. This scope check keeps you from making an address-list update for a problem that belongs in mail infrastructure.
When the recipient is the issue
For a clear recipient-not-found result, suppress the destination and ask the contact to provide a correction through an established channel. Do not guess a nearby username or automatically substitute another domain. Keep the original failure reason so an imported CSV does not silently reactivate the same destination later.
When policy is the issue
If the report points to authentication, authorization or sender policy, inspect the sender’s SPF, DKIM and DMARC setup and the provider’s diagnostic guidance. An address verifier cannot repair those settings. A valid recipient can still reject your message. Make one relevant configuration update, then test a narrow authorised sample before retrying a large batch.
Use probes cautiously
A verification probe does not reproduce your actual message, its headers or the exact sending infrastructure. Its response can therefore differ from a real delivery attempt. Use the bounce report to diagnose the failed send and the verifier to add address or routing evidence. Avoid claiming that every 550 from a probe conclusively proves that the mailbox is nonexistent.
Put it into practice
- Read code plus diagnostic text.
- Group failures by destination and sending stream.
- Suppress only when the recipient evidence supports it.
- Use provider guidance for sender-policy failures.