Soft bounce or hard bounce: the difference and what to do with each

A hard bounce means the address is permanently unreachable: it does not exist, or the domain does not exist. A soft bounce is temporary: mailbox full, server briefly unavailable, message too large. On a hard bounce you remove the address immediately; on a soft bounce you retry a limited number of times and only remove the address if the problem persists across several sends.

How to tell the difference: the error code

The receiving server answers with a three-digit code. Codes starting with 5 (for example 550 or 553) are permanent: the server says it will not work, not even later. Codes starting with 4 (for example 421 or 452) are temporary: try again later. Behind that code there is usually a more detailed status such as 5.1.1 (user unknown) or 4.2.2 (mailbox full), plus a text in plain language.

That text matters more than the code alone. A few examples from practice:

• 550 5.1.1 User unknown: hard. The address does not exist (any more). Remove.
• 550 5.1.2 Domain not found: hard. Typo in the domain or the company no longer exists. Remove.
• 452 4.2.2 Mailbox full: soft. The person has probably not been reading for a while. Retry, and remove on repetition.
• 421 4.7.0 Try again later: soft, but not about the address. The receiving server is slowing you down (greylisting or throttling). Waiting a moment and offering the message again usually solves it.
• 550 5.7.1 Message rejected due to policy: hard according to the code, but not about the address. This is a refusal based on reputation or authentication. Removing the address does not help; you need to look at your own side.

What to do with a hard bounce

Take the address out of your list, and make sure it does not come back through an import or a link with your CRM. That last part is often forgotten: the webshop removes the address from the newsletter, but the accounting package sends an invoice to it three months later. A central suppression list on the sending platform catches that, whichever application sends.

Why so strict? Receiving providers count how often you send to non-existent addresses. A sender who keeps doing that clearly does not maintain their list, and that lowers the deliverability of every message you send, including to addresses that are correct.

What to do with a soft bounce

Let the sending platform retry automatically. A common approach is a few attempts spread over one to a few days, with increasing intervals. If it still fails after that, the message is closed as undelivered. That is one soft bounce.

More important is what you do across several campaigns. An address that soft-bounces on three consecutive sends behaves in practice like a hard bounce: nobody reads there. So set a threshold, for example three or four consecutive soft bounces, and treat the address afterwards as if it had hard-bounced. Without that rule you drag the full mailboxes of departed employees along for years.

The bounce that is not an address problem

A third category is often misclassified: the block. If you see dozens of bounces from the same recipient domain within the same send, with a text about reputation, policy or a blocklist, those addresses are not dead. The recipient is refusing you. You must not remove those addresses; you must fix the cause on your side (authentication, IP reputation, sending pace) and then send again.

A sending platform that processes bounces automatically, as 2Mail does via IMAP, makes that distinction for you and shows you per refusal exactly what the receiving server answered. That saves you reading through hundreds of error messages by hand.

A practical example

A webshop sends a monthly newsletter to 8,000 addresses. After the send, 120 bounces appear in the overview. On closer inspection: 70 hard bounces with "user unknown" (old customers whose business address no longer exists), 35 soft bounces "mailbox full" and 15 refusals from one business domain with a policy text. The right response is threefold: remove the 70, flag the 35 and keep an eye on them over the next two sends, and for the 15 check whether the webshop's SPF record actually includes the sending server. Whoever simply deleted all 120 would throw away 15 good customers.

Frequently asked questions

How many bounces is normal?

That depends on the age of your list. A list you actively maintain stays well below what providers consider suspicious. If the number suddenly spikes, something has changed: an old import, or a block at one provider.

Should I also remove soft bounces for transactional mail?

No, not automatically. An order confirmation that hits a full mailbox three times is a customer you might do better to phone. With transactional email you decide case by case; with campaigns you let the threshold do the work.

Can I process bounces myself?

You can, but in practice it does not happen consistently. Let the platform do it and check the overview monthly.