What 550 5.1.1 means
5.1.1 is 'Bad destination mailbox address': the part before the @ isn't a valid mailbox at that domain. The domain itself is fine and accepts mail; that particular user doesn't exist. It's a permanent failure, and retrying will never work.
This is the classic hard bounce. Mailbox providers count invalid-recipient rates as a strong spam signal, since legitimate senders with opt-in lists rarely hit many of them. A cold campaign with a few percent of 5.1.1 bounces can damage a domain's reputation within days.
What the bounce looks like
The wording providers document for this code. Your bounce may add an IP address, a session ID or a help link after it.
550 5.1.1 The email account that you tried to reach does not exist. Please double-check the recipient's email address for typos or unnecessary spaces.550 5.1.1 Bad destination mailbox addressMicrosoft's documented description. Exchange NDRs often show it with a RESOLVER.ADR prefix.
554 delivery error: dd This user doesn't have a yahoo.com account ([address])Yahoo's documented example uses basic code 554 with no enhanced code; it means the same thing.
Why cold emailers see 550 5.1.1
- The person left the company and their mailbox was deleted. Lists go stale quickly; B2B data decays by a meaningful share every year.
- A guessed address pattern (first.last@ vs. flast@) that's wrong for that company.
- Typos or scraping errors in the lead data.
- Unverified lists from data vendors.
How to fix 550 5.1.1
- Suppress the address immediately and never send to it again from any inbox.
- Verify lists before every campaign, not once. Re-verify anything older than about 90 days.
- Be careful with catch-all domains: they accept everything at SMTP time and bounce later, so verifiers can't confirm them.
- Keep hard-bounce rate per campaign well under 2%. If a campaign exceeds it, pause it and re-verify the rest of the list before continuing.
Retry or suppress?
Suppress, permanently. Never retry a 5.1.1. Also stop any follow-ups queued for that contact.
Reading the code
The basic code 550 means "Mailbox unavailable or message refused (permanent)" under RFC 5321. In 5.1.1, the 5 is the class (permanent failure) and the 1 is the subject (addressing). The detail number 1 is registered in RFC 3463 / the IANA registry as "Bad destination mailbox address".
- Class 5
- Permanent failure
- Resending the same message won't work. Something about the message, the recipient or the sender has to change first.
- Subject 1
- Addressing
- A problem with the sender's or recipient's address: syntax or validity.
- Detail 5.1.1
- Bad destination mailbox address
Check the rest of your setup
Paste the headers of a message that did get through into the email header analyzer to confirm SPF, DKIM and DMARC pass, and check your domain with the SPF, DKIM & DMARC checker. The deliverability checklist covers the rest of a healthy sending setup.
Got a different bounce? Paste it into the SMTP error code lookup to decode it.