When an email cannot be delivered, the receiving server sends back a bounce message — a notification explaining why delivery failed. Understanding the two main types of bounces is essential for protecting your sender reputation and keeping your list healthy.
What is a Hard Bounce?
A hard bounce is a permanent delivery failure. The message cannot be delivered and will never be delivered to that address. Common causes include:
- The email address does not exist (user unknown)
- The domain no longer exists or has no mail server
- The recipient's server has permanently blocked your domain or IP
Action required: Remove hard-bounced addresses from your list immediately. Continuing to send to them signals to ISPs that you are not maintaining your list, which damages your sender score.
What is a Soft Bounce?
A soft bounce is a temporary delivery failure. The email address is valid, but the message could not be delivered at that moment. Common causes include:
- The recipient's mailbox is full (over quota)
- The receiving server is temporarily unavailable or overloaded
- The message is too large
- Temporary spam filter triggered
Action required: Your ESP will usually retry soft bounces automatically for 24–72 hours. If an address soft-bounces consistently over several sends, treat it like a hard bounce and remove it.
How Bounces Affect Sender Reputation
ISPs track your bounce rate across all your sends. The general thresholds to stay below are:
- Hard bounce rate: below 0.5% (ideally below 0.2%)
- Overall bounce rate: below 2%
Gmail in particular began enforcing stricter bounce rate thresholds in 2024 and started penalising high-bounce senders with spam folder placement or temporary blocks.
Preventing Bounces Before They Happen
The most effective strategy is to stop bad addresses from entering your list in the first place:
- Real-time verification on signup forms — Integrate an email verification API to check each address the moment it's entered. Invalid addresses are rejected before they're stored.
- Double opt-in — Require subscribers to confirm their email by clicking a link. This eliminates typos and fake addresses.
- Bulk list cleaning before sending — Upload your existing list to a verification service before any large send.
- Regular list hygiene — Remove unengaged contacts who haven't opened in 6–12 months. Many of these addresses become invalid over time.
Reading Bounce Codes
Bounce messages include numeric SMTP codes that describe the reason for failure:
| Code | Type | Meaning |
|---|---|---|
| 550 | Hard | User does not exist / mailbox unavailable |
| 551 | Hard | User not local; please try forwarding |
| 552 | Soft | Mailbox full / exceeded storage allocation |
| 421 | Soft | Service temporarily unavailable |
| 452 | Soft | Insufficient system storage |
Summary
Hard bounces demand immediate action — remove those addresses. Soft bounces need monitoring — retry automatically, but remove persistent soft-bouncers. The best protection against both is email verification before sending.