Bounce Codes Reference

Every time an email is sent, the receiving mail server responds with an SMTP status code indicating whether the message was accepted, temporarily deferred, or permanently rejected. Lettr surfaces these codes in your dashboard and via email.bounced webhook events, so understanding them helps you diagnose delivery issues quickly.

How SMTP Response Codes Work

SMTP response codes are three-digit numbers returned by a receiving mail server during or after the message transfer. The first digit determines the overall class of the response:

First Digit	Class	Meaning
2xx	Success	The server accepted the message for delivery.
4xx	Temporary Failure	The server could not process the message right now but may accept it later. These generate soft bounces.
5xx	Permanent Failure	The server will never accept this message. These generate hard bounces.

Modern mail servers also return Enhanced Status Codes (defined in RFC 3463) in the format x.y.z, such as 5.1.1. These provide more granular detail about the failure and are explained in a later section.

Success Codes

These codes indicate the message was accepted by the receiving server.

Code	Meaning
250	Message accepted for delivery. This is the standard success response.
251	User not local; the server is forwarding the message to another destination.

A 250 response means the receiving server accepted the message — it does not guarantee inbox placement. The server may still filter the message to spam or reject it at a later stage.

Temporary Failure Codes — Soft Bounces

Soft bounces are transient failures. Lettr automatically retries delivery for soft bounces according to its retry schedule. If retries are exhausted without success, the bounce may be reclassified as a hard bounce.

Code	Enhanced Code	Meaning	Recommended Action
421	4.7.0	Server temporarily unavailable or rate limiting	Auto-retry. Lettr handles this automatically.
450	4.2.1	Mailbox temporarily unavailable	Auto-retry. The mailbox may be locked or temporarily offline.
451	4.3.0	Server configuration issue on the receiving end	Auto-retry. Usually resolves on its own.
452	4.2.2	Mailbox full or over quota	Auto-retry. If the mailbox remains full across multiple attempts, this may be promoted to a hard bounce.
454	4.7.0	TLS negotiation failed; TLS not available	Retry without TLS if your sending policy allows opportunistic encryption.

Lettr retries soft bounces automatically with exponential backoff. You do not need to resend these messages manually. Monitor the Activity Log in your dashboard to track retry progress.

Permanent Failure Codes — Hard Bounces

Hard bounces are definitive rejections. The receiving server will not accept this message regardless of how many times it is sent. Lettr automatically adds hard-bounced addresses to your suppression list to protect your sender reputation.

Code	Enhanced Code	Meaning	Recommended Action
550	5.1.1	User unknown — mailbox does not exist	Remove the address from your list.
550	5.1.2	Bad destination domain — domain does not exist or has no MX records	Remove the address from your list.
550	5.2.1	Mailbox disabled or deactivated	Remove the address from your list.
550	5.7.1	Message rejected by policy (spam filter, blocklist, or authentication failure)	Check your content, authentication records (SPF/DKIM/DMARC), and sender reputation.
551	5.1.6	User not local and server will not forward	Remove the address from your list.
552	5.2.3	Message exceeds size limits	Reduce the message size (compress images, remove large attachments).
553	5.1.3	Invalid mailbox name syntax	Fix the email address format (check for typos or illegal characters).
554	5.7.1	Transaction failed — general rejection	Investigate the full bounce message for details. May indicate blocklisting or content issues.

Continuing to send to hard-bounced addresses damages your sender reputation and can lead to your IP or domain being blocklisted. Lettr suppresses these addresses automatically — do not remove them from your suppression list unless you have confirmed the address is valid again.

Enhanced Status Code Structure

Enhanced Status Codes follow the format x.y.z where each component has a specific meaning:

x — Class (matches the first digit of the basic SMTP code)
- 2 = Success
- 4 = Temporary failure
- 5 = Permanent failure
y — Subject category (what part of the system the issue relates to)
z — Detail (specific condition within that category)

Subject Categories

Subject (y)	Category	Description
0	Other / Undefined	Status does not fit other categories.
1	Addressing	Problems with the recipient or sender address.
2	Mailbox	Issues with the destination mailbox (full, disabled, not found).
3	Mail System	Problems with the destination mail system.
4	Network	Network-level issues (DNS, routing, connection).
5	Protocol	Protocol-related failures during the SMTP transaction.
6	Content	The message content triggered a rejection (encoding, media type).
7	Security	Security or policy violations (authentication, encryption, blocklisting).

For example, 5.1.1 means: permanent failure (5) related to addressing (.1.) with detail code 1 (mailbox does not exist).

Provider-Specific Codes

Major mailbox providers often return custom error messages alongside standard SMTP codes. Below are common patterns you may encounter.

Gmail

Error Message Pattern	Meaning	What to Do
`550 5.1.1 The email account that you tried to reach does not exist`	The Gmail address is invalid or has been deleted.	Remove the address from your list.
`421 4.7.28 ... rate limited`	Gmail is throttling your sending due to volume or reputation.	Reduce sending speed. Lettr retries automatically.
`550 5.7.1 Our system has detected that this message is likely unsolicited mail`	Gmail’s spam filters flagged your message.	Review your content, authentication, and list hygiene.
`550 5.7.26 Unauthenticated email ... is not accepted due to domain's DMARC policy`	Your message fails DMARC alignment.	Ensure SPF and DKIM are properly configured and aligned with your From domain.

Outlook / Microsoft 365

Error Message Pattern	Meaning	What to Do
`550 5.1.1 User unknown`	The Outlook/Hotmail address does not exist.	Remove the address from your list.
`550 5.7.1 Service unavailable, client host [x.x.x.x] blocked`	Your sending IP is on a Microsoft blocklist.	Check your IP against the Outlook SNDS portal and request delisting if needed.
`451 4.7.500 Server busy. Please try again later`	Microsoft is rate limiting connections.	Lettr retries automatically.
`550 5.5.0 Requested action not taken: mailbox unavailable`	Mailbox is disabled or deleted.	Remove the address from your list.

Yahoo / AOL

Error Message Pattern	Meaning	What to Do
`553 5.1.3 Invalid address`	The Yahoo/AOL address is malformed or does not exist.	Remove the address or fix formatting.
`421 4.7.0 [TS01] ... temporarily deferred`	Yahoo is throttling your messages due to sending patterns or reputation.	Reduce volume. Lettr retries automatically.
`554 5.7.9 Message not accepted for policy reasons`	Your message was rejected due to Yahoo’s content or authentication policies.	Review DKIM/SPF configuration and message content.
`421 4.7.1 [TS03] ... too many connections`	Too many simultaneous connections from your IP.	Lettr manages connection pooling automatically.

How Lettr Categorizes Bounces

Lettr maps raw SMTP response codes to standardized bounce categories that appear in your dashboard and in email.bounced webhook payloads. This makes it easier to take action without interpreting SMTP codes directly.

Lettr Category	SMTP Codes / Conditions	Description
`invalid_address`	550 5.1.1, 553 5.1.3	The recipient address does not exist or is malformed.
`domain_not_found`	550 5.1.2	The recipient domain does not exist or has no MX records.
`mailbox_full`	452 4.2.2, 552 5.2.2	The recipient mailbox is over its storage quota.
`blocked`	550 5.7.1 (IP/domain block), 554 5.7.1	The receiving server blocked delivery due to reputation or blocklisting.
`spam_related`	550 5.7.1 (content rejection)	The message was flagged as spam by the receiving server.
`policy_related`	550 5.7.1 (authentication), 551 5.1.6	Rejected due to authentication failure, DMARC policy, or other sending policies.
`server_error`	451 4.3.0, 421 4.7.0	A temporary server-side issue on the receiving end.
`protocol_error`	454 4.7.0, 552 5.2.3	A protocol-level failure such as TLS negotiation or message size limits.
`unknown`	Any unrecognized or ambiguous response	The bounce could not be classified into a specific category.

You can filter bounces by category in the Activity section of your dashboard, or use the category field in email.bounced webhook events to automate list management in your application.

Diagnosing Bounces

Step-by-step guide to investigating and resolving bounce issues.

Bounce Suppressions

How Lettr manages bounced addresses and suppression lists.

How Email Delivery Works

Understand the full email delivery pipeline from send to inbox.

Webhook Event Types

Reference for all webhook events including email.bounced payloads.

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Email Fundamentals

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Troubleshooting

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Glossary

Bounce Codes Reference

How SMTP Response Codes Work

Success Codes

Temporary Failure Codes — Soft Bounces

Permanent Failure Codes — Hard Bounces

Enhanced Status Code Structure

Subject Categories

Provider-Specific Codes

How Lettr Categorizes Bounces

Diagnosing Bounces

Bounce Suppressions

How Email Delivery Works

Webhook Event Types

​How SMTP Response Codes Work

​Success Codes

​Temporary Failure Codes — Soft Bounces

​Permanent Failure Codes — Hard Bounces

​Enhanced Status Code Structure

​Subject Categories

​Provider-Specific Codes

​How Lettr Categorizes Bounces

​Related Topics

Diagnosing Bounces

Bounce Suppressions

How Email Delivery Works

Webhook Event Types

How SMTP Response Codes Work

Success Codes

Temporary Failure Codes — Soft Bounces

Permanent Failure Codes — Hard Bounces

Enhanced Status Code Structure

Subject Categories

Provider-Specific Codes

How Lettr Categorizes Bounces

Related Topics