SMTP response received
550 5.7.1 Recipient Address Rejected: Email Address Marked as InvalidYou received this code because the recipient mailbox does not exist at the destination server. This is a permanent address failure. Unlike policy rejections, the address itself is bad: never created, deleted, or misspelled. The correct action is unambiguous — suppress the address immediately and stop attempting delivery.
📖 In this guide
👤 Who sees this code
- Bounce processing pipeline receiving DSN reports
- List manager investigating rising hard bounce rates
- Ops team building suppression list automation
🔧 What this guide fixes
- Confirm this is a true address failure, not a policy rejection in disguise
- Take the correct suppression action to protect sender reputation
- Prevent future 550 5.7.1 bounces through validation at collection
🗂 The 5.7.x security and policy family
550 5.7.1READINGDelivery not authorized · policy or auth failure
550 5.7.9Message content not supported by policy
550 5.7.26Multiple authentication failures
421 4.7.0Transient policy · often greylisting
⚡ Do these 3 things first (before diving deeper)
Add the address to your suppression list before anything else
550 5.7.1 is one of the few SMTP codes where the correct first action is unambiguous: suppress the address immediately. The recipient does not exist and no amount of retry, delay, or reconfiguration will change that. Add the address to your global suppression list which prevents future delivery attempts across all campaigns. Every retry to an invalid address is a wasted send that damages your reputation while producing zero value.
Check for typo patterns across recent list imports
One 550 5.7.1 is a bad address. Ten from the same import batch is a data quality problem. Common typo patterns cluster around popular domains: gmial.com, gnail.com, gmaill.com for Gmail; hotmial.com, homail.com for Hotmail; yhaoo.com, yaho.com, yahooo.com for Yahoo. If you see a spike, run the whole affected batch through validation before further sends. Pre-send validation costs a fraction of post-bounce cleanup.
Verify the code is really addressing, not policy in disguise
Some receivers return address-failure wording ("user unknown", "not found") with a 5.7.1 status code to hide policy rejections from senders. Read the exact enhanced status code, not just the descriptive text. If the code is 5.7.1, treat as policy (investigate sender-side). If the code is 5.1.x or 5.2.x, treat as address (suppress immediately). Confusing the two damages your list or your reputation in opposite directions.
Common causes of 550 5.7.1
- The mailbox was never created. The most common source of 550 5.7.1 is addresses that never existed in the first place. This happens with cold outreach lists purchased or scraped from low-quality sources, with automated generation of common patterns (info@, contact@, sales@) at target domains, and with typos at collection that neither the user nor the signup form caught. When 550 5.7.1 concentrates in specific import batches, the source itself is the problem.
- The mailbox was deleted or the account was closed. Addresses go stale. Users leave companies, cancel subscriptions, close providers, or die. Email accounts get deleted, either by the provider after long inactivity, or by the user during a personal migration. Lists older than 12 months typically contain 5 to 15 percent dead addresses that produce 550 5.7.1 on the next send. Scheduled list validation is the strongest defense.
- Typo in the address at collection. Human error at signup is unavoidable. The user types their address quickly, transposes letters, adds or drops a character, or picks the wrong domain suffix. Well-designed signup forms use client-side typo detection libraries like mailcheck.js that suggest corrections in real time. Forms without this protection let typos through to your database, where they wait until first send to reveal themselves as 550 5.7.1 bounces.
- Domain does not exist or has no MX records. Some receivers return address-failure codes when the recipient domain has no MX records or when the domain itself does not resolve. Technically these cases should return 5.1.2 (bad destination system) per RFC 3463, but implementations vary. If you see 550 5.7.1 concentrated in specific domains that never accept mail, verify the domain has valid MX records, and if not, suppress the whole domain in one operation.
- Role or generic address that was never provisioned. Cold outreach campaigns often target role-based addresses (admin@, info@, sales@, support@, contact@) at domains where those addresses were never created. This generates 550 5.7.1 at high rates and damages reputation quickly. It also risks hitting spam trap addresses that reputation providers seed under exactly these patterns. If your bounce rate on role-based sends exceeds 2 percent, the outreach strategy itself needs to change.
How to fix 550 5.7.1, step by step
- Add the address to global suppression immediately. The address is permanently invalid and every future send attempt will bounce with 550 5.7.1. Add it to your platform global suppression list, which is the state that prevents delivery attempts across all future campaigns, integrations, and API sends. This is not a punishment for the recipient — they never existed. This is protection for your sender reputation from your own infrastructure repeating a known-failing send.
- Identify the source of the bad address. Trace the address back to its source. Was it a form signup, a batch import, a manual entry, a purchased list, a scraped database? Each source has different failure modes and different remediation. If the address came from a specific batch that produced multiple bounces, the whole batch is suspect. If it came from a form, check whether that form uses typo detection. If it came from a purchased list, that vendor should be flagged as low quality.
- Audit similar addresses from the same source pre-emptively. Never wait for more bounces to reveal the scope of a bad batch. Once you identify a source producing 550 5.7.1s, run the entire batch through address validation before your next send. SMTPing validates across 13 signals (syntax, MX, SMTP handshake, catch-all detection, role account detection, disposable detection) and catches most invalid candidates before they generate a single bounce.
- Add typo detection to your signup forms. Prevent 550 5.7.1 at collection time by implementing client-side typo suggestion on your signup forms. Free JavaScript libraries like mailcheck.js compare submitted addresses to common domain patterns and suggest corrections when likely typos are detected. The library adds under 5KB to your page weight and catches the vast majority of gmial.com, yhaoo.com, and hotmial.com variants before they reach your database.
- Never remove suppression via automation. Some systems support automatic re-inclusion of hard-bounced addresses after a cooldown period, on the theory that mailboxes might come back. Never enable this for 550 5.7.1. The address was invalid when you sent, and the probability of it becoming valid later without an explicit opt-in is effectively zero. Automatic re-inclusion damages sender reputation by generating repeat bounces on known-bad addresses.
🌐 How each provider sends this exact code
⚖ How this code differs from adjacent ones
550 5.1.2550 5.2.1550 5.7.1Prevention going forward
550 5.7.1 is a list quality problem more than a technical problem. Preventing it means catching invalid addresses upstream of your send — at collection and during pre-send validation — rather than absorbing the reputation cost after the bounce. Our 550 5.1.1 User unknown hand-crafted guide covers the full state machine for permanent-address suppression, and our 550 5.2.1 Inactive account guide covers the retry-then-suppress pattern for adjacent inactive-account cases.
- Validate every new address at collection with SMTPing or equivalent. Adding validation to your signup form and API endpoints catches 90 percent or more of eventual bounces before they enter your database.
- Implement typo detection on signup forms with mailcheck.js or a similar library. This catches the specific subclass of address failures caused by human error, the largest single source in consumer-facing signup contexts.
- Re-validate lists older than 6 months before sending. Address quality decays over time as users abandon accounts and providers delete stale mailboxes. Quarterly re-validation catches this decay before it triggers bounces on campaigns to inactive segments.
- Never buy or scrape address lists. Purchased and scraped lists guarantee bounces at high rates (15 to 30 percent for scraped, 5 to 15 percent for purchased "validated" lists) and guarantee spam trap hits that damage reputation more than pure bounces.
- Follow suppression list best practices to prevent re-sends. Once an address is suppressed, keep it there, and make sure suppression enforces across all sending platforms, ESP integrations, and automation systems.
Frequently asked questions about 550 5.7.1
Should I suppress the address after a single 550 5.7.1?
What is the difference between 5.1.1 and 5.1.2?
Can 550 5.7.1 be a temporary problem that resolves later?
How do I distinguish an address failure from a policy 5.7.1 with "user unknown" wording?
What if the recipient later tells me they never got my email but the address is valid?
Do address-failure bounces affect my sender reputation?
Stop 550 5.7.1 bounces before they happen.
SMTPing catches invalid addresses, disposables, catch-all traps, role accounts and dead mailboxes before you send. Fewer bounces means less firefighting and a healthier sender reputation. 13 validation types, 25 free daily, no card required.
📚 Deep dive on related codes
About the Author
Alaa · LinkedIn
Email infrastructure specialist with 8+ years of hands-on experience in SMTP, deliverability, and email verification. I’ve configured and troubleshot mail systems across Postfix, Exchange, and cloud relays, managed IP reputation and warmup campaigns, and built verification pipelines processing millions of addresses. My work spans DNS authentication (SPF, DKIM, DMARC, BIMI), bounce handling, blocklist monitoring, and compliance frameworks including CAN-SPAM and GDPR. I write every article on SMTPedia to give email professionals, developers, and marketers the accurate, RFC-grounded reference they need.
About SMTPedia
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