Hard bounce vs soft bounce

Every bounce is a rejection message from the receiving mail server, but the reason matters enormously for what you should do next.

Hard bounce: permanent failure

A hard bounce (SMTP 5xx) means the address will never accept mail. Typical causes:

• The mailbox does not exist (550 5.1.1 — the most common)
• The domain does not exist or has no MX record
• The recipient server has blocked your domain outright

Action: suppress the address permanently, on the first bounce. Re-sending to a known hard bounce is one of the fastest ways to lose sender reputation.

Soft bounce: temporary failure

A soft bounce (SMTP 4xx) means "try again later". Typical causes:

• Mailbox full
• Receiving server down or overloaded
• Message too large
• Greylisting by the recipient's anti-spam layer

Action: your ESP retries automatically for 24–72 hours. Suppress the address after roughly 3–5 consecutive soft bounces across separate campaigns — persistent soft bouncing usually means an abandoned mailbox.

Reading the codes

• 550 / 5.1.1 — no such user. Hard.
• 550 5.7.1 — blocked by policy. Hard for that recipient; investigate your reputation.
• 452 / 4.2.2 — mailbox full. Soft.
• 421 4.7.0 — rate limited or greylisted. Soft.

What a healthy rate looks like

Keep hard bounces under 2% per send; under 0.5% is what well-maintained lists achieve. Anything above 5% risks throttling or suspension by your ESP. Track it with the numbers in the deliverability benchmarks.

Preventing bounces before they happen

Verification removes non-existent mailboxes before you send, which is what turns a 6% bounce rate into a sub-1% one. Verify at signup with the email verification api and clean existing lists with the bulk email checker.