Email Bounce Types

When an email can't be delivered, the receiving server sends back a bounce message explaining why. This isn't just an error notification—it's the email ecosystem's immune response. How you handle bounces determines whether you're treated as a legitimate sender or flagged as spam.

The difference between a legitimate sender and a spammer isn't the messages they send—it's how they respond when those messages bounce.

The Two Categories That Matter

Every bounce falls into one of two categories, indicated by the first digit of the SMTP error code:

5xx codes (Hard bounces): Permanent failures. Stop trying.

4xx codes (Soft bounces): Temporary failures. Try again later.

This distinction drives everything else.

Hard Bounces: The Door Is Permanently Closed

A hard bounce means the email address is invalid, doesn't exist, or will never accept your mail. The server is telling you clearly: this won't work.

User unknown (550 5.1.1): The mailbox doesn't exist. Maybe it never did, maybe it was deleted, maybe it's a typo. Either way, there's no one there to receive your message.

Domain doesn't exist (550 5.1.2): The domain in the email address has no MX records. You're trying to deliver mail to a building that isn't there.

Mailbox disabled (550 5.2.1): The mailbox exists but is closed—an employee who left, an account shut down for inactivity.

Blocked (550 5.7.1): The recipient server explicitly refuses mail from your IP or domain. You've been rejected at the door.

The only correct response to a hard bounce is immediate removal. Not tomorrow. Not after a few more tries. Now.

Continuing to send to hard bounced addresses is the behavioral signature of spam. Legitimate senders respect "no." Spammers don't. ISPs watch for this pattern.

Soft Bounces: Try Again Later

A soft bounce means the address is valid but something temporary prevented delivery. The server is saying "not right now" rather than "never."

Mailbox full (452 4.2.2): The recipient's inbox has hit its storage limit. This might resolve when they clean it out—or it might indicate an abandoned account.

Server issues (450 4.7.1): The receiving server is temporarily unavailable. Usually resolves within hours.

Greylisting (450 4.7.1): An anti-spam technique where servers reject first-time senders, expecting legitimate mail systems to retry. Spammers typically don't bother retrying, so this filters them out automatically.

Rate limiting (450 4.7.0): You're sending too fast. The server wants you to slow down.

Message too large (552 4.3.1): Your message exceeds the server's size limit. The only fix is a smaller message.

The correct response is automatic retry with exponential backoff:

• Wait 15 minutes, try again
• Wait 1 hour, try again
• Wait 4 hours, try again
• Continue for 4-5 days total

Most mail servers handle this automatically. If an address soft bounces repeatedly over several days (typically 3-7 attempts), convert it to a hard bounce and remove it. A temporarily full mailbox that stays full for a week is probably abandoned.

Block Bounces: You Have a Reputation Problem

Block bounces are a special category where delivery fails because you've been flagged:

IP blacklisted: Your sending IP appears on a blacklist like Spamhaus. Check your status, request delisting, and fix whatever got you listed.

Domain blocked: Your sending domain has a poor reputation with this provider. This requires building trust over time—you can't argue your way off a blocklist.

Content blocked: Something in your message triggered spam filters. Review your content for spam-trigger patterns.

Block bounces require investigation, not just retry logic. Something is wrong with your sending practices or reputation.

Bounce Rates: The Numbers That Matter

Hard bounce rate: (Hard bounces ÷ Total sent) × 100

• Target: Below 2%
• Acceptable: 2-5%
• Problematic: Above 5%

Total bounce rate: ((Hard + Soft bounces) ÷ Total sent) × 100

• Target: Below 5%
• Acceptable: 5-10%
• Problematic: Above 10%

High bounce rates tell ISPs your list is stale, purchased, or poorly maintained. This damages your sender reputation across all recipients, not just the bounced ones.

Prevention Is Better Than Handling

Double opt-in eliminates most bounces before they happen. When someone signs up, send a confirmation email. Only add them to your list when they click the confirmation link. This catches typos, fake addresses, and people who didn't actually want to subscribe.

Real-time validation at signup catches problems immediately:

• Syntax checking (is this a valid email format?)
• MX record verification (does this domain accept email?)
• Disposable email detection (is this a throwaway address?)

Sunset policies automatically remove subscribers who haven't engaged in 12-18 months. An address that never opens your emails will eventually become a hard bounce—or worse, a spam trap.

The Common Mistakes

Ignoring bounces entirely. Some senders just keep sending to the same list regardless of bounces. This destroys reputation faster than almost anything else.

Removing soft bounces too quickly. A single soft bounce is often temporary. Give it a few days and several retries before giving up.

Not removing hard bounces. Every hard bounce should be removed immediately. There's no "maybe it'll work next time" with hard bounces.

Treating all bounces the same. Hard and soft bounces require opposite responses. Conflating them guarantees you'll handle at least one type wrong.

No monitoring. If you're not tracking bounce rates, you won't notice the gradual decay until you're blacklisted.

Feedback Loops: Hearing About Complaints

Major ISPs offer feedback loops that notify you when recipients mark your messages as spam. This isn't technically a bounce, but it requires the same response: immediate removal.

Register for feedback loops with:

• Gmail (via Google Postmaster Tools)
• Microsoft (JMRP/SNDS)
• Yahoo (CFL program)

A spam complaint is worse than a hard bounce. The recipient actively told their email provider "I don't want this." Continuing to send after a complaint is the fastest way to damage your reputation.