At a glance. Hard bounce: Permanent: remove now · Soft bounce: Temporary: retried · Block bounce: Refused for policy: fix the cause · Guardrail: Hard bounces under about 2%
The short version
- What a bounce is. A bounce is a message that could not be delivered, reported back to the sender.
- Hard bounces. A hard bounce is a permanent failure.
- Soft bounces. A soft bounce is a temporary failure.
- Block bounces. A block bounce means the recipient's server refused your message because of who you are or what you sent, not because of the address.
- What rate is acceptable. Providers do not publish a bounce rate that is safe, and what is normal depends on how you collected your list.
- How to handle bounces. Let your sending platform record every bounce by recipient.
What a bounce is
A bounce is a message that could not be delivered, reported back to the sender. It can happen immediately, when the receiving server refuses the message during the conversation, or later, when a server accepts the message and then sends a failure notice.
Both carry a status code and some text, and together they tell you whether the problem is with the address, the recipient's mailbox, or with you.
Hard bounces
A hard bounce is a permanent failure. The classic cause is an address that does not exist, which comes back as a 550 reply with a 5.1.1 enhanced code, but it also covers a domain that does not exist and addresses the server will never accept.
Remove the address immediately and keep it removed. Mailing it again wastes a send and, repeated across many addresses, tells providers that you do not maintain your list.
Soft bounces
A soft bounce is a temporary failure. Common causes are a full mailbox, a server that is down or busy, a message that is too large, and an auto-reply configured to reject. The receiving server answers with a 4xx code, and your sending system keeps the message and retries.
After several attempts over a period of time the system gives up and reports a failure. If an address soft bounces across several sends in a row, treat it as dead.
Block bounces
A block bounce means the recipient's server refused your message because of who you are or what you sent, not because of the address. The reply is usually a 5.7.x code that mentions policy, a blocklist, authentication or reputation.
The address may be fine. Fix the cause: check SPF, DKIM and DMARC, check whether your IP or domain is listed, and look at your complaint rate. Resending the same message to the same server without a change will fail the same way.
What rate is acceptable
Providers do not publish a bounce rate that is safe, and what is normal depends on how you collected your list. A list built from confirmed sign-ups bounces very little, and a purchased or old list bounces a lot.
As a guardrail, we aim to keep hard bounces under about 2 percent and watch the trend. A sudden jump almost always means a bad import, a typo in a sign-up form or an old segment.
How to handle bounces
Let your sending platform record every bounce by recipient. Add hard bounces to a suppression list automatically so they are never mailed again, and review soft bounces for patterns by domain.
Prevent the avoidable ones: confirm new sign-ups with a double opt-in, validate addresses at the point of entry, and send to inactive subscribers only after asking whether they still want to hear from you.
Bounces and sender reputation
Mailbox providers watch how often you send to addresses that do not exist, because it is a sign of poor list sourcing. Hitting a spam trap, which is an address reserved to catch senders with bad lists, can do more damage than the bounce itself.
Good list hygiene is the cheapest deliverability work there is. Validate before you send, remove what bounces, and keep a record of when and how each person agreed to hear from you.
Further reading and sources
- RFC 5321: Simple Mail Transfer Protocol
- RFC 3464: An Extensible Message Format for Delivery Status Notifications
- Google: Email sender guidelines
Do it in MigoSMTP: help-center articles
Related guides and tools
- Email validator: Check addresses before you send.
- SMTP error codes explained: Read the reply codes.
- Bulk email sender guide: Requirements for larger senders.
- IP warm-up calculator: Ramp volume safely.
Want help planning your volume? Tell us what you send and we will suggest a setup.
Frequently asked questions
What is the difference between a hard and a soft bounce?
A hard bounce is a permanent failure, such as an address that does not exist, and the address should be removed at once. A soft bounce is a temporary failure, such as a full mailbox or a busy server, and the sending system retries it.
What is a good bounce rate?
There is no universal figure, but it should be low and steady. As a guardrail, we aim to keep hard bounces under about 2 percent of a send, and treat a rising trend as a reason to stop and clean the list.
Should I remove soft bounces?
Not after one. Your platform retries them, and removes an address after repeated failures over time. Remove an address from your active list if it keeps soft bouncing across several sends.
Why do I get bounces for addresses that used to work?
People leave jobs, close accounts and change domains, and addresses that have not been used for a long time can be turned into spam traps. Regular list cleaning and sunsetting of inactive subscribers prevents it.