Hard Bounce
What is Hard Bounce?
A permanent email delivery failure (`5xx` SMTP status such as `550 5.1.1 User Unknown` or non-existent domain NXDOMAIN) indicating that the destination address does not exist or will never accept mail.
Why Hard Bounce Matters in Production
Hard bounces are the fastest way to get an ESP account suspended or a sending domain blacklisted. Because legitimate opted-in contacts rarely hard-bounce on their first email, ISPs use your hard bounce rate as a primary detector for scraped, purchased, or bot-generated lists.
Engineering & Sender Best Practices
- • Never retry a hard-bounced email address—suppress it globally across your ESP, CRM, and outbound sequencers on the first `550` error.
- • Verify all imported CSVs before sending so `INVALID` addresses are stripped out before an MTA ever sees them.
- • Distinguish `550 5.1.1` (bad recipient) from `550 5.7.1` (policy/reputation block) when diagnosing delivery logs.
- • Gate signup forms with real-time verification so bots cannot trigger waves of hard bounces on your transactional domain.
Separating 550 5.1.1 Dead Mailboxes from 550 5.7.1 Reputation Blocks
An engineering team noticed a sudden surge in 550 errors when sending to Microsoft 365 corporate recipients. By inspecting the enhanced status codes, they realized 70% were 550 5.1.1 ("User Unknown" from stale CRM records) which triggered Microsoft SmartScreen to start returning 550 5.7.1 ("Delivery not authorized") across all remaining M365 recipients. Cleaning the CRM with MailVeri removed 14,500 dead 5.1.1 addresses and cleared the 5.7.1 block.
Technical Implementation & Wire Example
C: EHLO mail.yourdomain.com
S: 250-mx.google.com at your service
C: MAIL FROM:<[email protected]>
S: 250 2.1.0 OK
C: RCPT TO:<[email protected]>
S: 550-5.1.1 The email account that you tried to reach does not exist.
S: 550 5.1.1 Please try double-checking the recipient's email address for typos.MailVeri detects 550 5.1.1 rejections during pre-send verification and flags the address as INVALID before you ever hit send.
SMTP Reply Codes & Bounce Classification Reference
RFC 5321 & Enhanced RFC 3463 Status Codes| SMTP Code | Bounce Classification | Server Meaning | MailVeri Status & Action |
|---|---|---|---|
| 250 2.1.5 | Deliverable (2xx) | Recipient RCPT TO accepted by destination MTA | VALID (or CATCH_ALL if wildcard server) |
| 452 4.2.2 | Soft Bounce (Storage) | Recipient mailbox has exceeded storage quota | FULL_INBOX — pause sends for 30 days |
| 421 / 450 4.7.1 | Soft Bounce (Transient) | Temporary greylisting or concurrent connection deferral | UNKNOWN (0 credits billed) |
| 550 5.1.1 | Hard Bounce (5xx) | User unknown / mailbox does not exist on server | INVALID — suppress permanently |
| 550 5.7.1 | Policy / Reputation Block | Rejected due to sender reputation, RBL, or DMARC failure | Check sender domain DNSBL & authentication |
How to Fix 550 Hard Bounces & "550 5.7.1" Rejections →
Learn the difference between 550 5.1.1 unknown-user hard bounces and 550 5.7.1 reputation blocks—and how to fix both.
Free Bounces & SMTP Signals Tools & Workflows
Frequently Asked Questions About Hard Bounce
What is the difference between 550 5.1.1 and 550 5.7.1?
`550 5.1.1` means the recipient email address does not exist on the destination server (a bad address in your list). `550 5.7.1` means the recipient address may be valid, but the receiving server rejected your message due to your sender reputation, IP blacklist status, or failed DMARC authentication.
How many times should my ESP retry a 550 hard bounce?
Zero times. A 5xx status is a permanent fatal error under RFC 5321. Retrying a hard-bounced address wastes sending quota and signals to the ISP that you are ignoring bounce telemetry.
Why do B2B email addresses turn into hard bounces so quickly?
When employees change jobs, get promoted, or companies undergo layoffs and M&A domain migrations, IT administrators deactivate their Google Workspace or Microsoft 365 mailboxes—turning previously valid addresses into instant 550 5.1.1 hard bounces at a rate of ~18–22% per year.
Closely Related Terms
Explore All Email Verification Terms (A–Z)
Glossary Index →Ready to start verifying emails?
Get 1,000 free credits when you sign up. No credit card required.