Hard Bounce vs Soft Bounce: What They Mean and What to Do About Each

Troubleshooting

When an email can't be delivered, the receiving server sends back a bounce message. But not all bounces are the same. Some mean the address is permanently invalid, while others indicate a temporary problem that might resolve itself. Knowing the difference determines whether you should remove the address or try again.

What Is a Hard Bounce?

A hard bounce is a permanent delivery failure. The email will never be delivered to that address, no matter how many times you retry. The receiving server is telling you definitively that delivery is impossible.

Common Hard Bounce Causes

• Address doesn't exist — The mailbox was never created, was deleted, or the username is wrong (e.g., jhon@ instead of john@)
• Domain doesn't exist — The domain part of the address (after the @) has no mail server or DNS records
• Recipient server permanently rejects — The server explicitly refuses mail from your domain, usually due to policy or blacklisting
• Mailbox permanently disabled — The account was closed or deactivated by the provider

Hard Bounce Codes

Hard bounces typically return 5xx SMTP codes:

CodeMeaning 550Mailbox not found / does not exist 551User not local — address rejected 552Mailbox full and permanently over quota 553Mailbox name invalid 554Transaction failed (often policy-based rejection)

What to Do with Hard Bounces

Remove them immediately. There is no point retrying a hard bounce. Continuing to send to hard bounce addresses:

• Wastes your sending resources
• Signals to providers that you don't maintain your list
• Damages your sender reputation
• May trigger spam traps (recycled addresses that were once valid)

Most email service providers automatically suppress hard bounces. Verify yours does this, and never re-import addresses that have previously hard bounced.

What Is a Soft Bounce?

A soft bounce is a temporary delivery failure. The email couldn't be delivered right now, but might succeed later. The receiving server isn't permanently rejecting the message — it's saying "try again."

Common Soft Bounce Causes

• Mailbox full — The recipient's inbox has reached its storage limit
• Server temporarily unavailable — The receiving server is down or overloaded
• Message too large — The email exceeds the recipient's size limits
• Rate limiting — The receiving server is throttling your connections
• Temporary DNS issues — The receiving domain's mail servers are temporarily unreachable
• Content filtering deferral — The server wants to scan the message before accepting it

Soft Bounce Codes

Soft bounces typically return 4xx SMTP codes:

CodeMeaning 421Service not available — try again later 450Mailbox temporarily unavailable 451Server error — mail processing issue 452Insufficient storage

What to Do with Soft Bounces

Retry, but set limits. Most email systems automatically retry soft bounces for 24–72 hours. If the email still can't be delivered after multiple retries, it's treated as a permanent failure.

Your retry strategy should be:

1. First retry: Wait 15–30 minutes
2. Second retry: Wait 1–2 hours
3. Third retry: Wait 4–8 hours
4. After 72 hours: Stop retrying and treat as a delivery failure

If an address consistently soft bounces across multiple campaigns (not just one send), consider it a pattern rather than a fluke. Remove addresses that soft bounce 3–5 times consecutively.

How Bounces Affect Deliverability

Bounce rates directly influence your sender reputation. Email providers track what percentage of your messages bounce and use that data to decide whether your future emails reach the inbox.

Acceptable Bounce Rates

MetricTargetWarning Hard bounce rateBelow 0.5%Above 2% = serious problem Soft bounce rateBelow 2%Above 5% = investigate Total bounce rateBelow 2%Above 5% = list quality issue

A hard bounce rate above 2% strongly signals that you're sending to a poorly maintained list — possibly purchased, scraped, or simply never cleaned.

The Spam Trap Risk

When email addresses are abandoned, some providers recycle them as spam traps. The address stops bouncing and starts accepting mail again — but now it's a trap. Any sender mailing that address is flagged as having poor list hygiene.

This is why removing hard bounces quickly matters. An address that bounces today might become a spam trap in six months. If you keep it on your list and it suddenly "works" again, you've hit a trap.

Hard Bounce vs Soft Bounce: Quick Comparison

Hard BounceSoft Bounce TypePermanent failureTemporary failure SMTP codes5xx4xx Should retry?No — neverYes — with limits Remove from list?ImmediatelyAfter 3–5 consecutive failures Reputation impactHigh — signals bad listLow — if resolved quickly Common causeInvalid addressFull mailbox / server issue

How to Reduce Bounce Rates

Validate Before Sending

Use email verification tools to check addresses before adding them to your list. Verification catches:

• Typos and formatting errors
• Non-existent domains
• Known disposable email addresses
• Addresses likely to bounce

Use Double Opt-In

Require new subscribers to confirm their email address by clicking a link. This ensures:

• The address is real and receives email
• The person who signed up actually owns the address
• You have documented consent

Clean Your List Regularly

Schedule quarterly list reviews:

1. Remove all hard bounces (should be automatic)
2. Remove addresses that have soft bounced multiple consecutive campaigns
3. Remove subscribers with zero engagement for 6+ months
4. Run remaining addresses through a verification service

Monitor Bounce Rates Per Campaign

Track bounce rates for every send. A sudden spike often indicates:

• A bad list segment was imported
• A major email provider is having issues (temporary — wait and retry)
• Your sending IP or domain got blacklisted
• DNS changes broke your authentication

Check your email authentication and blacklist status if bounce rates spike unexpectedly.

What Your ESP Shows You

Most email service providers report bounces but categorize them differently. Common label variations:

• Mailchimp: "Hard bounce" and "Soft bounce" — auto-cleans hard bounces
• SendGrid: "Bounces" (hard) and "Blocks" (soft/policy) — separate suppression lists
• Klaviyo: "Bounced" with sub-categories — auto-suppresses after thresholds
• HubSpot: "Hard bounce" and "Soft bounce" — auto-suppresses hard bounces

Check your ESP's bounce handling settings. Most auto-suppress hard bounces, but soft bounce thresholds vary. Configure them to suppress after 3–5 consecutive soft bounces.