A hard bounce is a permanent delivery failure, usually a 5xx SMTP code, caused by a dead address, a non-existent domain, or an outright block. A soft bounce is a temporary failure, usually a 4xx code, caused by a full mailbox, a server that's down, or a message that's too big. The practical rule for soft bounce vs hard bounce: remove hard bounces from your list immediately, and retry soft bounces for 24-72 hours before giving up. The first digit of the SMTP code tells you which is which, 5 means permanent, 4 means temporary.
I've cleaned a lot of lists and watched a lot of bounce logs. Below is what each type means, the exact codes you'll see, and how I handle them in production.
What is a hard bounce?
A hard bounce means the mail server has decided your message will never be delivered to that address, no matter how many times you try. The recipient address or domain is permanently unreachable.
Common causes:
- The mailbox doesn't exist (typo, employee left, account deleted).
- The domain doesn't exist or has no mail server (bad MX record).
- The receiving server is permanently blocking your IP or domain.
- The address was fabricated, which is common on purchased or scraped lists.
Hard bounces are the dangerous ones for your reputation. A pile of "no such user" rejections is the single clearest signal to a spam filter that you're mailing a list you didn't earn. Suppress every hard bounce the moment it happens and never mail it again.
What is a soft bounce?
A soft bounce is a temporary failure. The address is real and the server is reachable, but something stopped delivery on this attempt. Try again later and it often goes through.
Common causes:
- Mailbox is full (over quota).
- Receiving server is down or overloaded.
- Message is too large for the recipient's limits.
- Greylisting (the server deliberately defers a first attempt from an unknown sender).
- Temporary rate limiting because you sent too fast.
Most sending systems retry soft bounces automatically for a day or two. The trap is the address that soft-bounces forever. An abandoned mailbox stays full permanently, so after 3-5 consecutive soft bounces I promote it to the suppression list and treat it like a hard bounce.
Soft bounce vs hard bounce: codes and handling
| Hard bounce | Soft bounce | |
|---|---|---|
| SMTP code range | 5xx (permanent) | 4xx (temporary) |
| Example codes | 550, 553, 554 | 421, 450, 452 |
| Typical cause | Dead address, bad domain, block | Full mailbox, server down, too large |
| Address valid? | No | Yes, usually |
| Action | Suppress immediately | Retry 24-72 hours |
| Retry attempts | Zero | 3-5 over 1-2 weeks |
| Reputation impact | High | Low (until persistent) |
Codes are 2026 conventions; exact wording varies by mail server.
How bounces affect deliverability
Mailbox providers watch your bounce rate as a proxy for list quality. Here's the rough scale I work to:
- Under 2 percent total bounces: healthy, no action needed.
- 2-5 percent: providers start throttling and deferring you. Clean the list.
- Over 5 percent: real risk of blocks and blacklisting.
Hard bounces weigh heavier than soft bounces because they prove the address never existed. Gmail and Microsoft both treat a spike in "no such user" rejections as evidence of a purchased or stale list, and they'll throttle inbound from your IP fast. If you're seeing high numbers, work through how to reduce email bounce rate before your next big send.
Two things keep bounces low at volume: verify addresses before you import them, and warm your IP slowly so receiving servers learn to trust you. A cold IP blasting full volume generates more soft bounces (deferrals and rate limits) than a warmed one. See how long IP warm-up takes for the schedule, and dedicated IP vs shared IP for why owning your reputation makes bounce handling predictable.
Practical handling rules
- Parse the SMTP code, not the human-readable text. The text varies wildly between servers; the 4xx/5xx code is reliable.
- Suppress 5xx hard bounces on the first hit.
- Retry 4xx soft bounces, but cap it. After 3-5 fails, suppress.
- Never re-mail a suppressed address, even months later. Reactivating dead addresses tanks reputation.
- Log everything. You want a per-address bounce history so you can spot the slow-soft-bounce-to-suppression pattern.
If you're on a relay that hides raw bounce codes from you, you're flying blind. That's a common reason people move off shared relays, covered in Amazon SES vs dedicated SMTP server and is managed SMTP worth it.
How BulkEmailSetup helps
Bounce handling is only as good as the infrastructure underneath it. BulkEmailSetup runs your sending on a dedicated SMTP server with your own warmed IP, so you get clean raw bounce codes, automatic suppression of hard bounces, and managed blacklist monitoring, none of the guesswork that comes with a shared relay. At sustained volume it's a flat monthly fee instead of per-email pricing. See pricing for current plans.
Frequently asked questions
What is the difference between a soft bounce and a hard bounce?
A hard bounce is a permanent failure (a 5xx SMTP code) caused by a dead or invalid address, a non-existent domain, or a block. You remove those addresses immediately. A soft bounce is a temporary failure (a 4xx code) like a full mailbox, a server that's down, or a message that's too large. You can retry a soft bounce for a few days before giving up.
Should I remove soft bounces from my list?
Not on the first soft bounce. Most mail servers and ESPs retry for 24-72 hours. If the same address soft-bounces on 3-5 consecutive sends over a week or two, treat it like a hard bounce and suppress it. Persistent soft bounces are usually abandoned mailboxes that stay full forever.
What SMTP codes indicate a hard bounce vs a soft bounce?
Codes in the 5xx range are hard bounces (550 'no such user', 553 'invalid recipient', 554 'blocked'). Codes in the 4xx range are soft bounces (421 'service unavailable', 450 'mailbox unavailable', 452 'insufficient storage'). The first digit tells you permanent (5) vs temporary (4).
What bounce rate is too high?
Keep your total bounce rate under 2 percent. Above 2 percent and mailbox providers start throttling you; above 5 percent you risk blocks and blacklisting. Hard bounces matter most because they signal a dirty or purchased list, which is exactly what spam filters look for.
