Bounced Email: Hard Bounces vs Soft Bounces

Every bounce is a signal. Ignore enough of them and inbox providers will start treating your domain as a spam source. Here is what each type means and how to respond.

What Is an Email Bounce?

An email bounce occurs when a message cannot be delivered and the receiving mail server returns an error response. The bounce carries an SMTP status code — a three-digit number that classifies the failure. Your ESP logs these and uses them to calculate your bounce rate.

Industry-accepted safe bounce rates sit below 2%. Rates above 5% trigger automated throttling or suspension by most ESPs.

Hard Bounces

Permanent delivery failure

SMTP 5xx codes — the server will never accept this message.

The mailbox does not exist (550 No such user here)
The domain has no valid MX record or has been decommissioned
The recipient's server has permanently blocked your sending IP
A typo in the address — missing TLD, transposed characters

Action required: Remove hard-bounced addresses immediately and permanently. Never retry them.

Soft Bounces

Temporary delivery failure

SMTP 4xx codes — try again later.

Mailbox is full (452 Insufficient system storage)
Receiving server is temporarily unavailable or overloaded
Message size exceeds recipient server limits
Greylisting — the server temporarily defers unknown senders

Retry strategy: Most ESPs retry over 24–72 hours. If an address soft-bounces 3+ consecutive campaigns, suppress it.

Why Bounces Hurt Sender Reputation

High bounce rates signal poor list hygiene to inbox providers — a characteristic shared with spammers. Consequences escalate: increased filtering, throttling, temporary blocks, then permanent blacklisting. See our sender reputation guide for the full picture.

Bounce Prevention Checklist

Verify every email address before adding it to your list — use FareOf's real-time API at point of capture.
Run a full list scrub every 90 days against your existing database.
Suppress all hard-bounced addresses permanently in your ESP.
Suppress addresses that soft-bounce 3+ consecutive times.
Never import purchased, scraped, or rented lists.
Use double opt-in for web form sign-ups to confirm address ownership.
Monitor your bounce report after every campaign and act within 24 hours.
Check your sending IP against major blacklists monthly.

Stop bounces before they start

Verify emails at point of capture or bulk-scrub your list in minutes.

Start free