Soft Bounce vs Hard Bounce: What to Do With Each

Every bounce is one of two kinds. A hard bounce means the address will never work. A soft bounce means it did not work this time. The distinction decides what you do next: remove the address, or retry and watch. Get it wrong in one direction and you keep mailing dead addresses; get it wrong in the other and you throw away live customers.

This guide gives you the definitions, the SMTP codes behind each, the rules we recommend for handling them, and the cases where a bounce looks hard but is not.

Hard bounce: permanent failure

A hard bounce is a permanent rejection. The receiving server has said, in effect, "do not try again". The SMTP reply code starts with 5.

Typical causes:

• The mailbox does not exist (550 5.1.1): a typo, a former employee, a closed account.
• The domain does not exist or has no mail server (550 5.1.2, or no MX record at all).
• The recipient server has blocked your address or domain by policy (550 5.7.1).
• The message was rejected as spam (550 5.7.1 or 554 5.7.1 with a spam-related explanation).

The first two are about the address and are the classic hard bounces. The last two are about you, and they are the ones to treat with care, as we explain below.

Soft bounce: temporary failure

A soft bounce is a temporary rejection. The address is valid, but the message could not be delivered right now. The SMTP reply code starts with 4, or the server accepted the message and later returned it.

Typical causes:

• The mailbox is full (452 4.2.2, or 552 5.2.2 on some servers, which is a 5xx code for a temporary condition).
• The server is busy or down (421 4.3.2, 451 4.3.0).
• Greylisting: the server deliberately refuses the first attempt (451 4.7.1 Please try again later).
• You are being rate limited (421 4.7.0, 451 4.7.650 from Microsoft).
• The message is too large for the mailbox.

Mail servers retry soft bounces on their own, usually for up to a few days, before giving up and reporting the failure to you.

How to handle each

Hard bounces from a bad address

Suppress the address immediately and do not import it anywhere else. There is no retry that will help, and every repeat send damages your reputation. Most sending platforms do this automatically, but suppression lists do not travel between tools.

Hard bounces caused by your setup

A 5.7.1 policy rejection is permanent for the message, not for the address. If Outlook blocks your IP, every Outlook address bounces; the mailboxes are fine. Keep the addresses, fix the cause, and reinstate any contacts your platform suppressed. We cover the most common case in why emails bounce from Outlook with 550 5.7.1.

Soft bounces

Let the retry run, then watch the pattern. One soft bounce means nothing. An address that soft-bounces on three or more consecutive campaigns over a few weeks is almost always dead: a full mailbox nobody empties is an abandoned mailbox. Treat it as a hard bounce at that point.

Many sending platforms apply this rule for you. Mailchimp, for example, converts an address to a hard bounce after repeated soft bounces. Check what your platform does and set the threshold if it lets you.

Benchmarks

Mailbox providers judge you mostly on hard bounces, because those show how careful you are with your list. Keep the total bounce rate under 2% per campaign. Between 2% and 5% is a warning; above 5% is where inbox placement starts to fall for the whole list. Our bounce rate calculator shows where a campaign sits, and the guide to reducing bounce rate covers what to change.

Soft bounces are rarely a reputation problem by themselves, but a sudden rise is a useful alarm. A jump in 4.7.x replies from one provider means you are being throttled or greylisted, and that is often the first sign of a reputation problem on its way.

Reading a bounce message

Bounces come back as a delivery status notification. The parts that matter:

When the Status says 5.x.x but the diagnostic text talks about policy, reputation, blocking or spam, the bounce is about you. When it says the user, mailbox or recipient is unknown, not found or does not exist, it is about the address.

Preventing bounces instead of sorting them

Hard bounces from bad addresses are the one category you can remove almost entirely before sending. An email verifier checks each address with the receiving server without sending a message, and reports the ones that would come back 5.1.1 as undeliverable. Full mailboxes and greylisting show up too, as Unknown, so you know which addresses to watch.

Verify lists before their first campaign, before mailing any segment that has been quiet for a few months, and always before importing a list you did not collect yourself. For how the check works, read how email verification works without sending an email.