What Is an Email Bounce?

A bounce occurs when an email you’ve sent is returned undelivered. The receiving server sends back a failure notification with a status code that tells you why the delivery failed. But not all bounces are the same — the difference between a hard bounce and a soft bounce determines what you should do next.

Hard Bounces: Permanent Failures

A hard bounce means the email address is permanently invalid or the domain doesn’t exist. Common causes:

  • The email address was never valid (typo at signup, like “gmal.com” instead of “gmail.com”)
  • The email account has been permanently deleted
  • The domain no longer exists or has expired

What to do: Remove hard bounces from your list immediately and permanently. Never retry a hard bounce — the address isn’t going to start working, and re-sending to known-invalid addresses tells ISPs that you don’t maintain your lists. A high hard bounce rate (above 2%) triggers spam filters across all major ISPs.

Soft Bounces: Temporary Failures

A soft bounce means the email couldn’t be delivered right now, but the address itself is valid. Common causes:

  • Mailbox full — The recipient’s inbox has hit its storage limit
  • Server temporarily unavailable — The receiving mail server is down for maintenance or experiencing issues
  • Message too large — Your email exceeds the recipient server’s size limit
  • Content rejection — The server’s content filter rejected the email (not the address itself)

What to do: Soft bounces can be retried. Most email sending platforms retry automatically 3–5 times over 72 hours. If an address soft-bounces repeatedly over multiple campaigns (typically 3–5 consecutive soft bounces), suppress it — it’s effectively unreachable.

Bounce Rate Thresholds

ISPs monitor bounce rates closely. Here are the thresholds you should stay under:

  • Hard bounce rate: Keep below 2% — above this, ISPs begin throttling or blocking your mail
  • Total bounce rate (hard + soft): Keep below 5%
  • Gmail specifically: Google recommends keeping hard bounces below 0.08% for consistent inbox placement

How to Prevent High Bounce Rates

The most effective prevention is email verification at the point of signup and before each campaign send.

  1. Real-time API verification — Verify email addresses as users sign up. Catch typos and invalid addresses before they enter your list.
  2. Bulk list verification — Run your entire list through a verification tool before each major campaign. Remove all Invalid results, and carefully evaluate Risky addresses.
  3. Double opt-in — Requiring subscribers to confirm their email address eliminates typos and guarantees the address is real and accessible.

Bounce Codes Reference

Email servers communicate bounce reasons via SMTP status codes:

  • 5xx codes — Permanent failures (hard bounces). Common: 550 (mailbox not found), 551 (user not local), 552 (mailbox full — sometimes treated as soft), 553 (invalid mailbox name)
  • 4xx codes — Temporary failures (soft bounces). Common: 421 (service temporarily unavailable), 450 (mailbox temporarily unavailable), 452 (insufficient storage)

MailPipe processes bounces automatically, updates your list in real time, and gives you a per-campaign breakdown of bounce codes so you can identify patterns. Start free today.