Bounce
What is Bounce?
When an email cannot be delivered to the recipient's address and is returned by the receiving mail server with an RFC 5321 / RFC 3463 error code. Bounces are categorized as "hard" (permanent 5xx failures like a non-existent mailbox) or "soft" (temporary 4xx deferrals like an over-quota inbox or busy server).
Why Bounce Matters in Production
Every bounce generates a negative telemetry signal at receiving MTAs (Gmail, Microsoft 365, Yahoo) and your sending ESP. While occasional soft bounces are normal, recurring hard bounces signal to spam filters that you are mailing unverified or scraped lists.
Engineering & Sender Best Practices
- • Parse enhanced SMTP status codes (e.g., 550 5.1.1 vs. 452 4.2.2) instead of treating all delivery failures identically.
- • Immediately add any 5xx hard-bounced address to your global suppression list across all sending tools.
- • Pause sends for 30 days to addresses returning 452 over-quota (`FULL_INBOX`) errors.
- • Verify lists with MailVeri before launching campaigns so permanent 550 failures are removed pre-send.
B2B Outbound & Transactional Webhook Triage
A SaaS platform sending 120,000 monthly lifecycle and outbound emails noticed a 3.4% total bounce rate in Postmark. By parsing enhanced RFC 3463 codes in bounce webhooks—separating 550 5.1.1 (permanent dead mailbox) from 452 4.2.2 (over-quota inbox) and 421 4.7.0 (temporary rate deferral)—and pre-verifying imported lists with MailVeri ($59 per 100K), the team eliminated 94% of avoidable 5xx failures before hitting send.
Technical Implementation & Wire Example
# Hard Bounce (Permanent 5xx Failure — Suppress Immediately)
550 5.1.1 <former.employee@company.com>: Recipient address rejected: User unknown
# Soft Bounce (Temporary 4xx Deferral — Over Quota / Full Inbox)
452 4.2.2 <shopper@example.org>: The email account that you tried to reach is over quotaReceiving mail servers distinguish permanent 5xx mailbox failures from temporary 4xx storage or rate-limit deferrals.
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 |
Email Bounce Codes Guide: 4xx vs. 5xx SMTP Errors Decoded →
Learn how to read traditional 3-digit and enhanced RFC 3463 SMTP bounce codes across Gmail, Outlook, and Proofpoint.
Free Bounces & SMTP Signals Tools & Workflows
Frequently Asked Questions About Bounce
What is the difference between a traditional 3-digit SMTP bounce code and an enhanced status code?
Traditional RFC 5321 codes use three digits (like 550 or 452), which can be ambiguous across providers. Enhanced RFC 3463 codes add a dotted X.Y.Z sub-code—such as 550 5.1.1 for an unknown user versus 550 5.7.1 for a policy or reputation block—so engineers can automate exact suppression vs. remediation rules.
Do bounces occur during the SMTP handshake or after the email is accepted?
Synchronous bounces occur during the live SMTP session (typically right after RCPT TO or after the DATA payload) and never leave your MTA queue. Asynchronous bounces (Non-Delivery Reports or NDRs) happen when a gateway like Proofpoint or a catch-all server initially returns 250 OK and later generates a bounce message when internal routing fails.
How does pre-send verification stop bounces without sending an actual email?
MailVeri performs a non-intrusive SMTP handshake (EHLO, MAIL FROM, RCPT TO) and disconnects with QUIT before the DATA stage, detecting 550 invalid mailboxes and 452 full inboxes silently for $0.0002–$0.0009 per check.
Closely Related Terms
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