Email deliverability glossary: 40 terms every sender should know
Forty email deliverability terms defined in one or two sentences each: bounces, SPF, DKIM, DMARC, spam traps, greylisting, sender reputation, inbox placement and verification.
Hard bounce vs soft bounce: what the difference means for you
A hard bounce is a permanent rejection (5xx), a soft bounce is temporary (4xx). What causes each, how ESPs handle them, and when a soft bounce should be treated as hard.
A hard bounce is a permanent delivery failure: the receiving server has said, with a 5xx SMTP code, that the message can never be delivered to that address, usually because the mailbox or domain does not exist. A soft bounce is a temporary failure signalled with a 4xx code: the address is valid but the message could not be delivered right now, because the mailbox is full, the server is busy, or the sender is being greylisted. Hard bounces should be removed from a list immediately; soft bounces can be retried, but an address that soft bounces three to five times in a row should be treated as hard.
What is a hard bounce?
When your ESP connects to the recipient's mail server and issues RCPT TO, a hard bounce is the server answering with a permanent negative reply. The most common is 550 5.1.1, "user unknown". Others include 550 5.1.2 (domain does not exist), 553 (invalid mailbox syntax) and 554 (transaction failed, often a policy block). The defining feature is the first digit: a 5 means "do not try again".
Some hard bounces never reach SMTP at all. If the domain has no MX record and no A record, there is no server to connect to, and the ESP records a bounce without a conversation. You can check what a domain publishes with the MX lookup, and the full list of codes we report is on the status codes page.
What is a soft bounce?
A soft bounce is a 4xx reply: the server has accepted that the address exists, or at least has not denied it, but cannot take the message now. 450 and 451 are the typical codes and they are almost always greylisting or a temporary server problem. 452 means the mailbox is over quota or the server is out of resources. Sending servers queue the message and retry, usually for 24 to 72 hours, before giving up.
The subtlety is that "temporary" is a statement about the code, not about reality. A mailbox that has been full for six months will soft bounce every message forever, and a server that greylists every attempt from your IP is effectively rejecting you. That is why the retry count matters.
Hard bounce vs soft bounce at a glance
| Hard bounce | Soft bounce | |
|---|---|---|
| SMTP reply | 5xx (550, 553, 554) | 4xx (450, 451, 452) |
| Permanent? | Yes | No, in principle |
| Typical cause | Mailbox or domain does not exist | Full mailbox, greylisting, busy server |
| Will a retry help? | No | Often, within minutes to hours |
| ESP behaviour | Suppressed immediately | Retried, then suppressed after repeated failures |
| Effect on reputation | Strong negative signal | Mild, unless it repeats |
| What you should do | Remove the address | Monitor; remove after 3 to 5 consecutive bounces |
What are the common reasons for a hard bounce?
- The person left. The most common cause on B2B lists. Work mailboxes are deleted within days or weeks of someone leaving, and every message after that is a 550.
- A typo at signup. gmial.com, hotmal.com, a missing letter in a name. Many of these resolve to domains that exist but have no mailbox by that name.
- A fake address. Someone typed asdf@asdf.com to get past a form.
- The domain expired. Small businesses close, projects end, and the domain either lapses or is bought by someone else.
- A policy rejection. The server blocks your sending IP or domain outright. The code is 5xx, so ESPs often count it as hard even though the address may be fine.
What are the common reasons for a soft bounce?
- Greylisting. The server rejects the first attempt from an unfamiliar sender with 450 or 451 and expects a retry a few minutes later. This is the single biggest cause of soft bounces and we explain it in greylisting explained for marketers.
- Mailbox full. Common on free providers with small quotas and on abandoned mailboxes that still receive newsletters.
- Message too large. The attachment or HTML exceeds the recipient's limit.
- Server overloaded or down. Maintenance windows, outages, or rate limiting of your sending IP.
- Content or reputation throttling. Some providers deliberately slow senders they are unsure about, returning 4xx codes until engagement data arrives.
How do ESPs handle each type?
Every mainstream ESP suppresses a hard bounce immediately: the address is marked as bounced, excluded from future sends, and usually cannot be re-added without an explicit reactivation. This is automatic and you should not fight it.
Soft bounces are retried by the ESP's own mail servers for a period, typically 24 to 72 hours, and only then recorded as a soft bounce against the contact. ESPs then apply a counter. Mailchimp, Klaviyo, Brevo and most others convert an address to a hard bounce, or "cleaned", once it has soft bounced a set number of times across consecutive campaigns. The exact number varies but three to five is the usual range, with some requiring the bounces to span a minimum number of days.
One catch: ESPs cannot see what caused a soft bounce once it is counted. A full mailbox and a greylisting delay look identical in the campaign report. If you want to know which it is, run the address through the email checker, which shows the SMTP reply and whether the mailbox was confirmed on retry.
When should a soft bounce be treated as hard?
The standard rule is three to five consecutive soft bounces, with no delivery in between, spread over at least two weeks. At that point the "temporary" problem has persisted through enough retries that the address is unlikely to recover, and continuing to send to it costs you reputation for no return.
Treat these cases as hard sooner:
- Mailbox full (452) on three sends in a row. A mailbox that nobody empties is a mailbox nobody reads.
- Soft bounces combined with zero engagement. If the address has never opened or clicked, do not wait for the fifth bounce.
- Soft bounces across an entire domain. If every address at one company starts bouncing, the domain has moved or died. Verify a few and suppress the rest if they fail.
A single soft bounce on an address that engaged last month is noise, and soft bounces that coincide with a provider-wide incident should be reset, not counted.
What should you do about bounces?
Hard bounces are a cleaning job, soft bounces are a monitoring job, and both are cheaper to prevent than to fix. Verify addresses at the form so typos and fakes never enter the list, verify any segment that has not been mailed in 90 days, and watch the hard bounce rate per campaign against the 2% ceiling. The steps are laid out in how to reduce your email bounce rate.
Key takeaways
- Hard bounce: 5xx reply, permanent, remove the address immediately.
- Soft bounce: 4xx reply, temporary, retried by the ESP for 24 to 72 hours.
- Greylisting and full mailboxes cause most soft bounces; dead mailboxes and typos cause most hard bounces.
- Treat an address as hard after three to five consecutive soft bounces, sooner if it has never engaged.
- Verification before sending removes most hard bounces and flags full mailboxes as risky.
Frequently asked questions
Does a soft bounce hurt sender reputation?
A little, and much less than a hard bounce. Repeated soft bounces to the same addresses do count against you, because they show you are sending to mailboxes that are not being read.
Can a hard bounced address come back?
Rarely. If the bounce was a policy block or a misconfigured server rather than a missing mailbox, the address may work again. Re-verify it before re-adding; most ESPs will not let you re-add a cleaned address without proof.
Why did a verified address hard bounce?
Usually because it was a catch-all domain that accepted the SMTP probe and rejected the message afterwards, or because the mailbox was deleted between verification and sending. Both are why a freshly verified list still bounces at 0.1% to 0.3%.
