Every email you send either reaches the inbox, lands in spam, or bounces. Bounces fall into two categories - hard and soft - and how you handle each one has a direct impact on your deliverability, your ESP standing, and ultimately your revenue.
This guide covers everything: what causes each type, how they affect your sender score, the SMTP codes that identify them, and the exact policy you should apply to each.
What Is a Hard Bounce?
A hard bounce is a permanent delivery failure. The message cannot be delivered now or in the future because the destination address or domain does not exist or has permanently rejected delivery.
Common causes of hard bounces:
- The email address doesn't exist (invalid mailbox)
- The domain has no MX records (no mail server configured)
- The domain no longer exists
- The receiving server has permanently blocked your IP or domain
- The address was deactivated (left a company, deleted their account)
550 5.1.1- User unknown, mailbox does not exist550 5.1.2- Domain not found / bad destination domain551 5.1.6- User not local; cannot relay553 5.1.3- Bad destination mailbox address syntax
SMTP codes that indicate hard bounces:
What to do: Remove hard bounced addresses from your list immediately and permanently. Never send to a hard bounce again. Most ESPs suppress hard bounced addresses automatically, but you should also remove them from your CRM, transactional email tools, and any other systems that might send to the same list.
What Is a Soft Bounce?
A soft bounce is a temporary delivery failure. The message could not be delivered right now, but the address itself is valid and may accept delivery in the future.
Common causes of soft bounces:
- The recipient's inbox is full (over quota)
- The receiving mail server is temporarily unavailable or overloaded
- The message is too large for the server's limits
- The server responded with a temporary deferral (greylisting)
- Rate limiting: the receiving server accepted your connection but deferred the message
421 4.7.0- Server temporarily unavailable450 4.2.2- Mailbox over quota / unavailable451 4.3.0- Temporary system problem452 4.2.2- Mailbox full, message temporarily rejected
SMTP codes that indicate soft bounces:
What to do: Most ESPs automatically retry soft bounces for 24-72 hours. You should track how many times an address soft bounces and apply a policy: typically, if an address soft bounces on 3 consecutive campaigns, reclassify it as a hard bounce and remove it. A valid address rarely stays unavailable across multiple separate sends.
The Key Differences: Hard vs Soft Bounce
| Hard Bounce | Soft Bounce | |
|---|---|---|
| Cause | Permanent failure | Temporary failure |
| Address validity | Invalid or blocked | Valid but unavailable |
| Action | Remove immediately | Retry and monitor |
| ESP impact | Immediate reputation damage | Minimal if infrequent |
| SMTP range | 5xx codes | 4xx codes |
| Retry | Never | Automatic (24-72h) |
The 5xx vs 4xx SMTP code convention is standardized: 5xx codes indicate permanent errors, 4xx codes indicate temporary errors that may self-resolve.
How Bounces Affect Your Sender Reputation
Your sender reputation - the score that determines whether your email lands in the inbox or spam folder - is directly impacted by your bounce rate.
Hard bounces are the most damaging signal. A single campaign with a 5% hard bounce rate can trigger automatic ESP suspension and flag your sending IP with major mailbox providers like Gmail, Outlook, and Yahoo.
Industry thresholds to know:
- 2% hard bounce rate: Most ESPs flag or pause your account
- 5% hard bounce rate: Automatic suspension by most ESPs
- Any bounce above 1% is a warning sign worth investigating
Soft bounces matter less individually, but patterns matter. An address that soft bounces repeatedly tells you the mailbox is effectively unreachable - and sending to it repeatedly generates the same reputation signals as sending to an invalid address.
Google's Postmaster Tools tracks your domain reputation with Gmail. If you see your domain reputation drop from High to Medium or Low, high bounce rates are often the cause.
The Bounce Handling Policy You Should Use
Here is the bounce handling policy we recommend based on analysis of millions of email sends:
For hard bounces:
For soft bounces:
Why 3 consecutive? A valid address may temporarily be unavailable (vacation autoresponder, temporarily full inbox, server maintenance). But if the same address soft bounces on three separate campaigns sent weeks apart, the probability of it ever being deliverable drops below 20%.
How to Prevent Bounces Before They Happen
The best bounce handling is prevention. Two strategies eliminate the majority of bounces before they occur:
1. Email verification at signup
Integrate a real-time email verification API at your sign-up form. This checks syntax, MX records, and SMTP deliverability before accepting the address. Invalid addresses are blocked before they ever enter your database. BounceZero's API adds sub-100ms verification to any sign-up flow.
2. Bulk list verification before campaigns
Email lists decay at approximately 22.5% per year - about 2% per month. Before any major campaign, verify your full list with a bulk email verification service. This catches addresses that have become invalid since they were collected.
Addresses that pass bulk verification but still occasionally soft bounce are typically temporary inbox issues - not list hygiene problems. Addresses that fail bulk verification would have hard bounced on your campaign and damaged your reputation.
Bounce Rate Benchmarks by Industry (2026)
Average hard bounce rates vary significantly by industry and list quality:
| Industry | Average Hard Bounce Rate |
|---|---|
| Software/SaaS | 0.3-0.8% |
| E-commerce | 0.5-1.2% |
| B2B / Professional services | 0.8-1.5% |
| Non-profit | 0.4-0.9% |
| Healthcare | 0.7-1.4% |
| Real estate | 1.0-2.0% |
| Retail | 0.5-1.0% |
Higher rates in B2B and real estate reflect the naturally higher churn in corporate email addresses (people change jobs frequently, company domains change). This is why B2B email verification needs to run more frequently than B2C verification.
Frequently Asked Questions
What is the difference between a hard bounce and a soft bounce?
A hard bounce is a permanent delivery failure - the address doesn't exist or the domain has no mail server. A soft bounce is temporary - the mailbox was full, the server was unavailable, or the message was deferred. Hard bounces must be removed immediately; soft bounces can be retried.
How many times should I retry a soft bounce?
Most ESPs retry soft bounces automatically for 24-72 hours. Across campaigns, if an address soft bounces on 3 consecutive sends, treat it as a hard bounce and remove it. An address that can't receive email across three separate attempts is effectively invalid.
Does a 2% bounce rate really cause ESP suspension?
Yes. Most major ESPs - Mailchimp, Klaviyo, ActiveCampaign, SendGrid, and others - will pause or suspend your account if your hard bounce rate exceeds 2% on a single campaign. The threshold exists to protect their shared sending infrastructure and the reputation of all senders using their platform.
Can I prevent bounces with email verification?
Yes. Email verification at signup blocks invalid addresses from entering your list, eliminating most future bounces at the source. Bulk list verification before campaigns catches addresses that became invalid since collection. Together, these two practices keep most senders well below a 1% bounce rate.
What is the best SMTP code to identify a hard bounce?
550 5.1.1 (User Unknown) is the most common hard bounce code, indicating the mailbox does not exist. Other permanent 5xx codes (551, 553, 554) also indicate hard bounces. Temporary 4xx codes (421, 450, 451, 452) indicate soft bounces.