Understanding Email Bounce Codes: What They Mean and How to Respond
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What Is a Bounce Code?
When an email cannot be delivered, the receiving mail server sends back a response explaining why. This response includes a status code and usually a human-readable message. These are bounce codes, and understanding them helps you decide what to do with undeliverable addresses on your list.
Bounce codes matter for extracted email lists because those lists have not been pre-validated. Addresses pulled from old files, archived emails or exported records may include addresses that no longer exist, are full, or are blocked. When you send to these addresses and they bounce, the codes tell you why and what to do next.
Hard Bounces vs. Soft Bounces
Bounces fall into two broad categories:
Hard bounces are permanent delivery failures. The address does not exist, the domain is invalid, or the server has permanently rejected your message. Hard-bounced addresses should be removed from your list immediately. Continuing to send to them damages your sender reputation.
Soft bounces are temporary delivery failures. The recipient's mailbox is full, the server is temporarily unavailable, or the message is too large. Soft-bounced addresses may accept mail in the future. You can retry, but if the same address soft-bounces repeatedly (three or more times across different sends), treat it as a hard bounce and remove it.
Common Bounce Codes
Email bounce responses use SMTP status codes. The most common ones you will encounter:
5xx codes (hard bounces)
550 - User not found / Mailbox unavailable. The email address does not exist on the receiving server. This is the most common hard bounce. The address may have been closed, deactivated, or was never valid. Remove it from your list.
551 - User not local. The receiving server does not recognise the recipient. Similar to 550 in practice. Remove the address.
552 - Message size exceeded. The message (including attachments) is larger than the recipient's server allows. This is technically a rejection of the message rather than the address, but it prevents delivery. Reduce the message size and retry.
553 - Mailbox name not allowed. The address format is invalid or the server rejects the specific mailbox name. Remove it.
554 - Transaction failed / Policy rejection. A general rejection, often due to content filtering, blacklisting or policy violations. This can mean the receiving server has blocked your sending IP or domain. Check your sender reputation and whether you are on any blacklists.
4xx codes (soft bounces)
421 - Service not available. The receiving server is temporarily down or too busy to accept connections. Retry later. Most email platforms retry automatically.
450 - Mailbox busy. The recipient's mailbox is temporarily locked or in use. Retry later.
451 - Local error in processing. The receiving server encountered a temporary internal error. Retry later.
452 - Insufficient system storage. The receiving server is out of storage space. This affects all recipients on that server, not just one address. Retry later.
Enhanced status codes
Many servers return enhanced status codes in the format X.Y.Z (for example, 5.1.1). These provide more detail:
5.1.1 - Bad destination mailbox address. The specific mailbox does not exist. Hard bounce. Remove.
5.1.2 - Bad destination system address. The domain part of the email address does not exist or has no mail server. Hard bounce. Remove.
5.2.1 - Mailbox disabled. The account exists but has been disabled or suspended. Hard bounce. Remove.
5.2.2 - Mailbox full. The recipient's mailbox is over its storage quota. Soft bounce. The address may accept mail once the recipient clears space. Retry, but remove after repeated failures.
5.7.1 - Delivery not authorised. The receiving server has rejected your message based on policy. This can mean your IP is blocked, your domain fails SPF/DKIM/DMARC checks, or the content triggered a filter. Investigate your sending configuration.
4.2.2 - Mailbox full (temporary). Same as 5.2.2 but explicitly marked as temporary. Retry.
4.7.1 - Greylisting. The receiving server is temporarily rejecting your message because it has not seen your sending IP before. It expects legitimate senders to retry. Most email platforms handle this automatically.
What to Do When You Get Bounces
After sending to an extracted list
If you send to a list of extracted email addresses and receive bounces:
- Separate hard bounces from soft bounces. Your email marketing platform typically does this automatically and labels each bounce.
- Remove hard bounces immediately. Add them to your suppression list so they are not included in future sends.
- Monitor soft bounces. If the same address soft bounces on two or three consecutive sends, move it to your suppression list.
- Check for patterns. If many addresses on the same domain bounce, the domain may be defunct or may be blocking your messages. A cluster of 5.7.1 rejections from one domain suggests a policy issue on their end or a reputation issue on yours.
Reducing bounces before sending
The best way to handle bounces is to prevent them. Before sending to an extracted list:
- Validate the list. Run addresses through an email validation service to identify invalid, inactive and risky addresses before you send. Email Extractor does not validate addresses. See What is email validation.
- Check for obvious problems. Look for addresses with typos in common domains (gmial.com, yaho.com), addresses with invalid characters, and role-based addresses like info@ and admin@ that are more likely to reject marketing messages.
- Start small. Send to a small batch first and check your bounce rate before sending to the full list. A bounce rate above 2% on the first batch is a warning sign that the list needs more cleaning. See Email warm-up explained.
Bounce rate thresholds
Most email service providers set bounce rate thresholds:
- Under 2%. Acceptable. Normal list maintenance will keep you here.
- 2% to 5%. Elevated. Clean your list and investigate the source of bad addresses.
- Over 5%. Critical. Your sending reputation is at risk. Pause sending, clean the list thoroughly, and validate all remaining addresses before resuming.
If you consistently exceed your provider's threshold, they may suspend your account.
How This Connects to Extracted Lists
Extracted email lists are especially prone to bounces because:
- Old files contain old addresses. Addresses from documents that are months or years old may have been closed, abandoned, or reassigned.
- No prior validation. Unlike addresses collected through a sign-up form with double opt-in, extracted addresses have never been confirmed as deliverable.
- Mixed sources. Extraction pulls every email address from a file. Some may be system addresses, defunct accounts, or addresses that were never intended to receive messages.
Validation before sending is the most effective way to reduce bounces. The email list audit checklist covers the full review process.
