A hard bounce is a permanent delivery failure: the address or its domain does not exist, so the email will never arrive and the address should come off your list today. A soft bounce is a temporary failure, like a full inbox or a busy server, and the email may still arrive if it is retried.
The code tells you which one you have: 5.x.x is permanent, 4.x.x is temporary.
Below are the codes you will actually see from Gmail and Microsoft, what each one means, and what to do with the contact, plus the third kind of bounce most guides skip.
What is the difference between a hard bounce and a soft bounce?
The difference between a hard bounce and a soft bounce is whether the failure can fix itself. A hard bounce is permanent: the mailbox or domain does not exist, and resending will fail every time. A soft bounce is temporary: the mailbox exists, but something (a full inbox, a rate limit, a server delay) stopped delivery this time.
Mail servers say which one it is in the status code. The Enhanced Mail System Status Codes standard (RFC 3463) defines 5.x.x as a permanent failure “not likely to be resolved by resending”, and 4.x.x as a transient failure where “sending in the future may be successful”.
Hard bounce: what causes it and what to do
A hard bounce means the receiving server looked for the mailbox or the domain and it was not there. The common causes on a B2B list:
- The person left the company and IT deleted the mailbox.
- The address was guessed from a naming pattern and the pattern was wrong.
- A typo in the name or the domain.
- The domain changed after a rebrand or an acquisition, or expired.
- The list is old. ZeroBounce reports that at least 23% of an email list degrades within a year, based on 11 billion addresses it checked in 2025. More on how fast B2B data decays.
The codes you will see most are 550 5.1.1 (Gmail: “The email account that you tried to reach does not exist”), 550 5.1.10 (Microsoft 365: recipient not found), 553 5.1.2 (the domain could not be found) and 550 5.2.1 (the account is inactive).
What to do: remove the address from every list and sequence the same day. If the person matters, find their current address or verify an email address before you try again, or call them.
Soft bounce: what causes it and what to do
A soft bounce means the mailbox is probably real but could not take the message right now. The common causes:
- The inbox is full. Gmail answers 452 4.2.2: “The recipient’s inbox is out of storage space.”
- You sent too fast. Gmail answers 421 4.7.28 when it sees “an unusual rate of email” and rate-limits you for a while.
- Greylisting. Some servers reject unfamiliar senders once with a 4xx on purpose and accept the retry (RFC 6647).
- The server was down or timed out. Your server keeps retrying, and reports a failure only when it gives up.
What to do: let your sending tool retry. If the same address soft bounces on three separate sends in a row, treat it as a hard bounce and remove it. That is our rule of thumb.
Tools set their own limits. Mailchimp, for example, converts a soft bounce to a hard bounce after 7 soft bounces for a contact with no activity, or 15 for one with past activity (Mailchimp).
The third kind: blocks (5.7.x)
A block is a permanent-looking 5.x.x code that is about your sending, not the address. The mailbox exists; the server refused the message because of who sent it or how.
- 550 5.7.1: Gmail judged the message “likely unsolicited email” and blocked it.
- 550 5.7.26: Gmail blocked an unauthenticated sender. Gmail requires every sender to authenticate with SPF or DKIM.
- 550 5.7.515: Outlook.com rejects mail from domains sending more than 5,000 emails a day that fail its SPF, DKIM and DMARC requirements, since May 2025 (Microsoft).
What to do: do not delete the contact. Fix the sender: authentication, volume, content and list quality. Our email deliverability best practices cover the sender side. Then check how your tool labels these codes, because many count them as hard bounces and suppress good contacts.
How to read a bounce code
Read the first digit, then the middle number. That tells you which of the three buckets a bounce belongs to and what to do with it.
The reference table below covers the codes you will meet most often from Gmail and Microsoft 365.
Example: sorting the bounces from one campaign
Marcus Reed, an SDR at Keystone Payroll in Philadelphia, sends a 400-email first touch to HR directors. The next morning he has 11 bounces.
| Code | Count | Type | Marcus’s action |
|---|---|---|---|
| 550 5.1.1 | 7 | Hard | Removes all 7 today |
| 553 5.1.2 | 1 | Hard | Removes it; checks if the company changed domains |
| 452 4.2.2 | 2 | Soft | Leaves them in; retries on the next step |
| 550 5.7.1 | 1 | Block | Keeps the contact; reviews his copy and sending volume |
His sending tool reports a 2.75% bounce rate (11 of 400), above Instantly’s 2% line. But only 8 are hard bounces (2.0%), and all 8 came from contacts that were never verified. The fix is upstream: verify the next list before it is loaded.
Common challenges with email bounces
Bounces rarely arrive as neatly as the table suggests. Four problems come up again and again.
A soft bounce that never clears
A mailbox that is full for weeks is usually abandoned. Gmail even has a separate permanent code for it: 552 5.2.2, “out of storage space and inactive”. Fix: cap retries, and move repeat soft bounces to your removal list.
The bounce arrives days after the send
A catch-all domain accepts every address at send time, then bounces the ones it cannot deliver. RFC 5321 requires a server that fails after accepting a message to send a failure notice back. Fix: check bounces again 48 to 72 hours after each send, and handle catch-all emails in a separate batch.
Your tool labels the same code differently
One tool counts a 5.7.1 as a hard bounce, another as a block, a third as a soft bounce. Fix: open the raw bounce message and read the code yourself before you delete anyone.
A spike right after an import
A sudden jump in hard bounces almost always traces back to one old or unverified list. Fix: pause that campaign, verify the rest of the list, and resume with only confirmed addresses.
Best practices to prevent hard bounces
Most hard bounces are preventable, because they come from addresses that were already dead before you sent. The fix starts upstream, when you are finding business contact information.
Two of these habits come with hard numbers:
- Authentication. Google requires SPF, DKIM and DMARC from anyone sending more than 5,000 messages a day to personal Gmail accounts, and every sender needs SPF or DKIM.
- Bounce rate. Hunter’s 2025 data, from 31 million emails sent by its users, puts the average cold email bounce rate at 3.6%. Instantly’s 2026 benchmark report advises staying under 2%. Our email bounce rate guide shows how to measure yours.
How Scalelist helps you avoid hard bounces
Scalelist, an AI Lead Finder, verifies every email before it returns it and labels it Valid or Risky, so dead addresses are caught before they reach your sequencer. Scalelist finds and verifies contact details on demand rather than serving a pre-compiled list.
- Before you send: the Scalelist Email Verifier checks addresses you already have: syntax, domain, MX, mailbox, catch-all, disposable and spam-trap signals. Verification is included in every plan.
- When you build a list: Lead Finder and the Email Finder return emails already labeled Valid or Risky (what each status means). You pay 1 credit per email found, Valid or Risky, and nothing when no email is found.
- After a hard bounce: reach the same person by phone. The Scalelist Mobile Finder charges 20 credits per mobile found, 0 on a miss.
New accounts get 20 free credits, no credit card required, and paid plans start at 29 USD a month (Scalelist pricing).
Check your next list before it bounces: every email comes back Valid or Risky.
Get 20 credits to test itFAQ
What is a hard bounce?
A hard bounce is an email that failed permanently because the recipient’s address or domain does not exist or is no longer active. It comes with a 5.x.x status code, such as 550 5.1.1 (“the email account does not exist”). Resending will fail every time, so remove the address from all lists and sequences the same day.
What is a soft bounce?
A soft bounce is a temporary delivery failure: the mailbox exists but could not accept the message right now. Common causes are a full inbox (452 4.2.2 at Gmail), rate limiting (421 4.7.28) and greylisting. It carries a 4.x.x code. Your sending tool retries it automatically, and the next attempt often succeeds.
Should I remove soft bounces from my list?
Not after one. A single soft bounce usually clears on the next attempt. Remove an address when it keeps soft bouncing: our rule of thumb is three separate sends in a row. Tools set their own limits; Mailchimp, for example, converts a soft bounce to a hard bounce after 7 soft bounces, or 15 if the contact has past activity.
Is a 5.7.1 error a hard bounce?
Not in the usual sense. A 5.7.1 is a permanent code, but it means the server refused the message because of the sender, its content or its reputation, not because the mailbox is missing. Keep the contact, and fix authentication, volume and content first. Deleting contacts over 5.7.x blocks throws away real people.
What is a good email bounce rate?
As low as you can get it, and under 2% is a common line: Instantly’s 2026 benchmark report advises keeping bounce rates below it. Hunter’s 2025 data from 31 million emails puts the average cold email bounce rate at 3.6%, so an unverified list will often sit above that line.
Can a hard-bounced address start working again?
Sometimes: a deleted mailbox can be recreated, or a new employee can get the same address. But sending to it again on the chance is not worth the risk. If the contact matters, verify the address again first, or reach the person another way, such as a phone call.
Do bounces hurt email deliverability?
Yes. Mailbox providers read a high bounce rate as a sign that a sender does not maintain its list, and that can push later emails, even to good addresses, toward spam. Microsoft lists bounce management among its recommended practices for high-volume senders. Remove hard bounces fast and verify lists before you send.
Sources
- RFC 3463: Enhanced Mail System Status Codes
- RFC 5321: Simple Mail Transfer Protocol
- RFC 6647: Email Greylisting
- Google Workspace: Gmail SMTP errors and codes
- Google: Email sender guidelines
- Google: Email sender guidelines FAQ
- Microsoft Learn: Fix NDR error 550 5.1.10 in Exchange Online
- Microsoft: Outlook’s new requirements for high-volume senders
- Mailchimp: Soft vs. hard bounces
- Hunter: The State of Cold Email
- Instantly: Cold Email Benchmark Report 2026
- ZeroBounce: The Email List Decay Report for 2026
