Diagnosing Email Bounces: Soft (4xx) vs Hard (5xx)

Soft Bounces vs Hard Bounces

When an email can't be delivered, the receiving server returns an SMTP reply code explaining why. These bounce codes fall into two categories:

Soft bounce (4xx) — Temporary failure. The server might accept the message later.
Hard bounce (5xx) — Permanent failure. The message will never be delivered.

Common Soft Bounces (4xx)

421 — Service Not Available

The receiving server is temporarily refusing connections. Causes:

Server is overloaded
Rate limiting on incoming connections
Temporary greylisting

Action: Your mail server should retry automatically (typically for 48-72 hours).

450 — Mailbox Temporarily Unavailable

The mailbox exists but can't accept mail right now. Often caused by:

Greylisting (deliberate delay for first-time senders)
Mailbox locked by another process

Action: Retry. Greylisting resolves on the second attempt.

452 — Insufficient Storage

The recipient's mailbox is full.

Action: Retry for a few days. If persistent, the address may be abandoned.

Common Hard Bounces (5xx)

550 — Mailbox Not Found

The email address does not exist. The most common hard bounce.

Action: Remove the address from your list immediately. Continuing to send to invalid addresses damages your sender reputation.

551 — User Not Local

The server doesn't handle mail for this user.

Action: Remove from list or check if the address is correct.

552 — Message Size Exceeded

The message (including attachments) exceeds the server's size limit.

Action: Reduce attachment size. Host files externally and link to them.

553 — Mailbox Name Not Allowed

The email address format is invalid.

Action: Validate the address format and remove if invalid.

554 — Transaction Failed

A catch-all rejection. Often means:

Content flagged as spam
IP address is blacklisted
SPF/DKIM/DMARC authentication failure

Action: Check your content, IP reputation, and authentication records.

Enhanced Status Codes (RFC 3463)

Many servers include enhanced codes in the format X.Y.Z:

5.1.1 — Bad destination mailbox address
5.2.2 — Mailbox full
5.7.1 — Delivery not authorized (policy rejection)
4.7.0 — Temporary rate limit
4.4.1 — Connection timeout

List Hygiene Best Practices

Remove hard bounces immediately
After 3 soft bounces across different sends, treat as hard bounce
Validate email addresses at sign-up (confirmation email)
Use a bounce processing webhook from your ESP (SendGrid, SES)
Monitor your bounce rate — above 2% signals reputation risk

Reading Bounce Messages

The bounce email (DSN — Delivery Status Notification) contains:

The SMTP reply code (e.g., 550)
Enhanced status code (e.g., 5.1.1)
Human-readable explanation from the receiving server
Original message headers for reference