Email Bounces Explained: Hard vs Soft and Every Bounce Type

Bounce Basics

When you send an email, the receiving server either accepts it or rejects it. A rejection is a bounce. The type of bounce depends on the SMTP response code the server returns.

Understanding the difference matters because your handling strategy depends entirely on the bounce type. Handle them wrong and you'll either damage your reputation (by continuing to send to dead addresses) or lose valid subscribers (by removing temporary failures).

Hard Bounces (5xx)

Hard bounces are permanent. The receiving server is telling you the delivery cannot succeed, now or ever.

Common hard bounce codes:

Code	Meaning	Action
550	User not found / mailbox doesn't exist	Remove immediately
551	User not local, no forwarding	Remove immediately
552	Mailbox storage exceeded (permanent)	Remove after 30 days
553	Invalid mailbox name/syntax	Remove immediately
554	Transaction failed / permanent block	Investigate, likely remove

Causes:

• The email address doesn't exist (typo, employee left, account deleted)
• The domain doesn't exist or has no MX records
• The receiving server has permanently blocked your sending IP or domain

Practitioner note: The most common hard bounce I see is the "left the company" bounce — addresses that were valid when collected but the person is gone. This is why B2B lists degrade fast. Expect 2-3% annual attrition from employee turnover alone.

Soft Bounces (4xx)

Soft bounces are temporary. The server is saying "not right now, try again later."

Common soft bounce codes:

Code	Meaning	Action
421	Service not available, try later	Auto-retry
450	Mailbox busy	Auto-retry
451	Local processing error / rate limited	Auto-retry, check reputation
452	Insufficient storage	Auto-retry, remove if persistent

Causes:

• Recipient's mailbox is full
• Receiving server is temporarily down
• You're being rate-limited (sending too fast)
• Greylisting (intentional temporary rejection to test retry behavior)
• DNS lookup failures on the receiving end

Most ESPs retry soft bounces automatically over 24-72 hours before converting them to permanent failures.

Block Bounces

Some bounces aren't about the address — they're about you. These are content or reputation-based rejections:

Code	Meaning	Action
550 (with block message)	IP/domain blacklisted	Check blacklists
554 5.7.1	Message rejected, policy reasons	Check authentication, content
421 4.7.0	Temporary rate limit from ISP	Slow down, check reputation

Block bounces require investigation, not just address removal. If you're getting blocked at Gmail or Outlook, the problem is your reputation or authentication, not the recipient address.

Practitioner note: When a client tells me "we're bouncing at Gmail," the first thing I check is Google Postmaster Tools. Nine times out of ten, it's a reputation issue triggering 421 deferrals, not actual bounces. The distinction matters — removing those addresses won't fix the problem.

Bounce Handling Strategy

Immediate Actions

• Hard bounces: Remove the address from your list after the first occurrence. Never retry.
• Soft bounces: Let your ESP retry automatically (most retry 3-5 times over 24-72 hours).
• Block bounces: Investigate the cause. Check authentication, reputation, and blacklist status.

Ongoing Management

• Track bounce rate per campaign: Watch for sudden spikes
• Track bounce rate per segment: New subscribers bouncing? Your signup form might be collecting junk.
• Convert persistent soft bounces: If an address soft-bounces across 3-5 consecutive sends, treat it as hard

Red Flags

Pattern	Likely Cause	Fix
Sudden spike in hard bounces	List has aged addresses or purchased contacts	Audit list source
High soft bounces to one domain	Server issues or you're rate-limited	Throttle sends, check reputation
Block bounces from Gmail	Domain reputation drop	Check Postmaster Tools
Bounces on previously valid addresses	Employee turnover or domain expiration	Regular list cleaning

Practitioner note: One pattern that catches people off guard: a domain you've been sending to successfully suddenly starts hard-bouncing everything. The company let their domain expire or migrated to a new domain. Their old MX records are gone. It's not your problem, but you need to catch and remove these quickly before you rack up bounce rate penalties.

Impact on Deliverability

High bounce rates directly hurt your sender reputation:

• ISP perspective: High bounces signal you're sending to an unmaintained list
• Blacklist perspective: Sending to many non-existent addresses suggests you're using a scraped or purchased list
• ESP perspective: Most ESPs will suspend accounts with bounce rates above 5-10%

Keep your total bounce rate below 2% per send. Hard bounce rate should stay under 0.5%. If you're exceeding these, your list needs a hygiene overhaul — check our list hygiene guides for step-by-step cleaning processes.

If your bounce rates are elevated and you're not sure why, schedule a deliverability audit — I'll trace the bounces to their source and fix the underlying problem.

Sources

• RFC 5321: Simple Mail Transfer Protocol
• RFC 3463: Enhanced Status Codes for SMTP
• Google: Email Sender Guidelines
• M3AAWG: Senders Best Common Practices
• Validity: State of Email Report

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v1.0 · April 2026