Email Bounce Management and SMTP Codes: Complete Classification and Suppression Guide

Email bounces fall into two categories with completely different operational responses: hard bounces (permanent failures) require immediate and permanent suppression; soft bounces (temporary failures) require intelligent retry logic. Getting this classification wrong in either direction damages deliverability — suppressing too aggressively removes valid addresses from your list, suppressing too loosely sends to invalid addresses that damage IP reputation and generate spam trap hits.

Hard Bounces vs Soft Bounces: The Fundamental Distinction

Hard bounce (permanent failure): The destination mail server definitively rejected the message with a 5xx SMTP error indicating the address, domain, or policy is permanently invalid. Examples: the email address doesn't exist, the domain has no MX records, the receiving server has a permanent policy block. Action: suppress immediately and permanently. Never retry.

Soft bounce (temporary failure): The destination mail server rejected the message with a 4xx SMTP error indicating a temporary condition. Examples: mailbox full, server temporarily unavailable, rate limit exceeded. Action: retry on a backoff schedule. After multiple consecutive soft bounces over 5–7 days, convert to suppression.

The distinction is encoded in the SMTP response code prefix: 5xx = permanent, 4xx = temporary.

SMTP Error Code Taxonomy

SMTP error codes follow a three-digit format (XYZ) where the first digit indicates permanent (5) or temporary (4) failure, the second indicates the general category, and the third provides detail.

5xx hard bounce codes

# 5.1.1 — Bad destination mailbox address (address doesn't exist)
550 5.1.1 The email account that you tried to reach does not exist.

# 5.1.2 — Bad destination mailbox address domain (domain doesn't exist/have MX)
550 5.1.2 We weren't able to find the recipient domain.

# 5.1.3 — Bad destination mailbox address syntax (malformed address)
550 5.1.3 The address format is illegal.

# 5.4.1 — No answer from host (domain exists but not accepting email)
550 5.4.1 Recipient address rejected: Access denied.

# 5.7.1 — Delivery not authorised (policy block)
550 5.7.1 [x.x.x.x] The IP address sending this message does not have a
  good reputation.

# 5.7.26 — Gmail authentication failure
550 5.7.26 This message does not have authentication information.

# 5.7.28 — Gmail new sender policy
550 5.7.28 The SMTP server requires that you authenticate via a security
  policy set by the domain owner.

# 5.7.515 — Microsoft authentication failure (May 2025)
550 5.7.515 Access denied, sending domain does not meet authentication standards.

4xx soft bounce codes

# 4.2.1 — Mailbox busy or temporarily unavailable
421 4.2.1 The user you are trying to contact is receiving mail too quickly.

# 4.2.2 — Mailbox full
452 4.2.2 Your message exceeded Google's message size limits.

# 4.3.0 — Mail system full (infrastructure issue)
421 4.3.0 Please try again later.

# 4.4.2 — Connection dropped or timed out
421 4.4.2 Connection reset. j8si12345678.

# 4.7.0 — Security policy (Gmail rate limiting)
421 4.7.0 [IP] Our system has detected an unusual rate of unsolicited mail.

# 4.7.650 — Microsoft reputation throttling
421 4.7.650 The mail server [IP] has exceeded the maximum number of
  connections.

# TS01 — Yahoo complaint rate throttling
421 4.7.1 [TS01] Messages from x.x.x.x temporarily deferred due to user complaints.

ISP-Specific Error Codes That Matter

Gmail diagnostic codes

• 5.7.26 — Authentication missing or failing. Fix SPF/DKIM/DMARC before any other action.
• 4.7.0 (421) — Rate limiting due to unusual volume or complaint patterns. Back off; do not retry aggressively.
• 5.7.1 with "Unsolicited mail" — Spam classification. Content or reputation issue.
• 4.2.1 — Per-user rate limit (sending too many messages to a single Gmail address too fast)

Microsoft Outlook codes

• 5.7.515 — Authentication failure (May 2025 requirement). Hard bounce — authentication is required to fix before retrying.
• 5.7.606 — IP block listed in Microsoft's blocklist. Submit to sender.office.com for review.
• 4.7.650 (421) — Connection rate exceeded. Reduce concurrent connections to Microsoft MXs.
• 5.7.1 with "blocked" — IP is on Microsoft's internal blocklist. Use sender.office.com removal portal.

Yahoo/AOL codes

• TS01 (421) — Complaint rate too high. Reduce sending, process FBL complaints, segment to engaged only.
• TS02 (421) — IP reputation generally poor. Reputation improvement required.
• TS03 (421) — Connection rate exceeded. Not a reputation issue — reduce concurrent connections to Yahoo MXs.
• 554 delivery error — Permanent Yahoo rejection. Usually indicates the specific mailbox is full or deactivated.

Bounce Classification Logic

Automated bounce processing requires classification logic that maps SMTP codes to operational actions:

# Classification rules (PowerMTA bounce-action format)
# Hard suppress: definitive non-existent address
5.1.1 → suppress permanently (bad-mailbox)
5.1.2 → suppress permanently (bad-domain)
5.1.3 → suppress permanently (bad-syntax)
5.4.1 → investigate (may be temporary block or permanent policy)

# Policy blocks: investigate before suppressing
5.7.1 → flag for review (policy, may resolve with reputation improvement)
5.7.26 → fix authentication, not a list issue — do not suppress the address
5.7.515 → fix authentication — do not suppress

# Soft bounce: retry with backoff
4.x.x → retry every 4 hours, max 3 days; convert to soft-suppress after 3 days

# TS codes: rate limit, not list issue
4.7.1 TS01/TS02/TS03 → retry after ISP-specified delay; do not suppress

Suppression Policies: What to Suppress and When

Immediate permanent suppression: Any 5.1.1, 5.1.2, 5.1.3, or 5.1.6 response — these definitively mean the address is invalid. Suppress on first occurrence.

Soft-suppress after extended soft bouncing: An address that generates 4xx soft bounces for more than 5 consecutive days should be soft-suppressed (removed from regular sends, re-tested monthly). If a monthly test still produces soft bounces for 60 days, convert to permanent suppression.

FBL complaint suppression: Immediate and permanent on first complaint receipt. No exceptions, no evaluation of whether the complaint was "fair."

Manual unsubscribe: Process within 10 business days (CAN-SPAM) or immediately (best practice). Add to global suppression list, not just a single campaign exclusion.

Do NOT suppress for: 421 TS-code throttling, 4.7.0 Gmail rate limiting, authentication-related 5.7.x codes (these indicate your infrastructure problem, not an invalid address), or one-time 4xx temporary failures.

Bounce Rate Thresholds by ISP

Bounce Handling in PowerMTA and Postfix

# PowerMTA: bounce-action rules in /etc/pmta/config
<bounce-action>
  type permanent          # Hard bounce
  match bad-mailbox bad-domain routing-errors
  action suppress         # Add to suppression list automatically
</bounce-action>

<bounce-action>
  type transient          # Soft bounce
  match quota-issues
  action retry
  retry-after 4h
  max-tries 18            # ~3 days at 4-hour intervals
</bounce-action>

<bounce-action>
  type transient
  match spam-related
  action retry
  retry-after 6h
  max-tries 12            # 3 days at 6-hour intervals
  # If still failing after 12 tries, message is held
</bounce-action>

# Postfix: soft bounce behaviour (main.cf)
# Retry interval for deferred messages
minimal_backoff_time = 300s    # 5 minutes minimum
maximal_backoff_time = 4000s   # ~67 minutes maximum

# Maximum time to keep trying before bouncing sender
maximal_queue_lifetime = 3d    # Bounce after 3 days of failures

# Soft bounce classification (prevents hard bounce on temp errors)
soft_bounce = no    # Set yes only in testing environments