What is SMTP error 550?

SMTP 550 means the receiving server permanently rejected your email. Common causes include an invalid recipient address, poor sender reputation, a blocked IP address, or failed authentication (SPF, DKIM, or DMARC).

What does SMTP 421 mean?

421 is a temporary server error. The receiving server is unavailable or limiting connections. Most email systems automatically retry delivery, so no immediate action is usually needed.

What is the difference between a hard bounce and a soft bounce?

Hard bounces are permanent failures such as invalid email addresses or domain rejection. Soft bounces are temporary problems such as full mailboxes, greylisting, or server outages. Hard bounces should be removed from your list. Soft bounces can be retried.

Can SMTP errors be fixed?

Most SMTP errors can be resolved by fixing authentication configuration, improving sender reputation, correcting recipient addresses, or waiting for temporary server issues to clear. Permanent (5xx) errors require action. Temporary (4xx) errors often resolve on their own.

What is the most common SMTP error?

SMTP 550 is the most common rejection code. It covers many specific issues including invalid recipients, relay denial, SPF or DKIM failures, IP blacklisting, and policy rejections. Always read the full error message after the 550 code for the specific cause.

47 Reasons Your Email Was Rejected (Complete SMTP Error Guide)

Few things are more frustrating than clicking send only to receive an SMTP error. Whether you are sending newsletters, transactional emails, or one-to-one messages, SMTP response codes explain exactly why an email was accepted, delayed, or rejected. Some errors are temporary and resolve themselves. Others require immediate action to prevent future delivery problems.

This guide explains 47 of the most common SMTP errors you will encounter and how to fix them.

Understanding SMTP Errors

SMTP uses three-digit response codes. The first digit indicates the nature of the response.

Code Type	Meaning	Action Required
2xx	Success	None
4xx	Temporary failure (soft bounce)	Retry later
5xx	Permanent failure (hard bounce)	Fix something first

Complete SMTP Error Reference Table

#	SMTP Code	Meaning
1	421	Service unavailable
2	421 Too many connections	Rate limited
3	421 Timeout	Connection timed out
4	450	Mailbox unavailable
5	450 Busy mailbox	Recipient unavailable
6	451	Local server error
7	451 Greylisted	Temporary block
8	451 Spam detected	Temporary filtering
9	452	Mailbox full
10	452 Storage exceeded	Quota reached
11	452 Too many recipients	Recipient limit exceeded
12	500	Syntax error
13	501	Invalid address syntax
14	502	Command not implemented
15	503	Bad command sequence
16	504	Unsupported parameter
17	521	Server does not accept mail
18	530	Authentication required
19	534	Authentication mechanism rejected
20	535	Authentication failed
21	538	Encryption required
22	541	Message rejected
23	550	Mailbox unavailable
24	550 User unknown	Recipient missing
25	550 Relay denied	Relay not permitted
26	550 Domain blocked	Policy rejection
27	550 SPF failed	Authentication failure
28	550 DKIM failed	Signature invalid
29	550 DMARC failed	Policy failed
30	550 Blacklisted	IP or domain blocked
31	551	User not local
32	552	Message too large
33	552 Storage exceeded	Mailbox quota
34	553 Invalid recipient	Bad address
35	553 Sender rejected	Sender blocked
36	554 Transaction failed	Generic rejection
37	554 Spam content	Spam filters triggered
38	554 Policy violation	Provider rules
39	554 IP reputation	Poor sender reputation
40	554 Virus detected	Malware found
41	554 Content rejected	Unsafe content
42	554 Too many complaints	Complaint threshold exceeded
43	554 DNS failure	DNS issues
44	554 Reverse DNS failed	Missing PTR record
45	554 TLS failure	Encryption problem
46	554 Rate limit exceeded	Sending too quickly
47	554 Unknown rejection	Server-specific failure

Most Common SMTP Errors Explained

421 Service Unavailable

The receiving server is temporarily unable to accept mail. Common causes include server maintenance, too many simultaneous connections, or a temporary outage. Most email providers automatically retry delivery, so no immediate action is needed unless the error persists for more than 24 hours.

450 Mailbox Unavailable

The recipient mailbox cannot currently receive mail. This usually means the mailbox is locked, the recipient is offline, or a temporary issue is affecting that specific account. It normally resolves without intervention.

451 Local Error

A temporary problem occurred on the receiving server. This can be caused by greylisting (a spam reduction technique that temporarily rejects unknown senders), a DNS lookup failure, internal anti-spam systems, or server resource issues. If the error persists for more than a few hours, investigate your authentication setup and sending reputation.

452 Insufficient Storage

The recipient mailbox is full. The recipient must clear space before new email can be delivered. There is nothing you can do on your end except wait or notify the recipient through another channel.

500 and 501 Syntax Errors

These indicate malformed SMTP commands or invalid email addresses. Check the recipient address for typos, verify your SMTP configuration, and review your email formatting for issues that could cause parsing errors.

530 Authentication Required

Your SMTP server requires authentication before it will accept the email for delivery. This is usually fixed by configuring the correct SMTP username and password in your email client or application.

535 Authentication Failed

Your login credentials were rejected by the SMTP server. Check your username, password, SMTP port, and SSL or TLS settings. If you recently changed your password, update it in all applications that send email through that server.

550 Mailbox Unavailable

This is the most common SMTP rejection by a wide margin. The 550 code is a catch-all for many specific problems, and it is critical to read the full bounce message that follows the code.

550 Variant	Likely Cause
550 User unknown	The recipient address does not exist
550 Relay denied	Your server is not authorised to relay through this provider
550 Domain blocked	The recipient domain has blocked your sending domain
550 SPF failed	Your SPF record does not include the sending IP
550 DKIM failed	The DKIM signature is missing or invalid
550 DMARC failed	DMARC policy rejected unauthenticated mail
550 Blacklisted	Your IP or domain is on a blocklist

552 Message Too Large

Your email exceeded the receiving provider's size limit. Reduce attachment sizes, compress or remove embedded images, and minimise HTML weight. Most providers limit emails to 10 MB to 25 MB total including attachments.

553 Invalid Recipient

The recipient address is invalid or incorrectly formatted. This usually means a typo in the email address, an unsupported special character, or a domain that does not exist. Verify the spelling before resending.

554 Transaction Failed

The 554 code is a generic rejection that covers many specific issues. It nearly always requires investigation rather than a simple retry.

554 Variant	Likely Cause
554 Spam content	The email triggered spam filters
554 Policy violation	The provider's terms prohibit this type of email
554 IP reputation	Your IP address has poor sending reputation
554 Virus detected	Malware or suspicious attachments found
554 Too many complaints	Your complaint rate exceeds the provider's threshold
554 DNS failure	The receiving server cannot resolve your domain
554 Reverse DNS failed	Your IP lacks a PTR record matching your domain
554 TLS failure	Encryption negotiation failed
554 Rate limit exceeded	You are sending faster than the provider allows

Soft Bounce vs Hard Bounce

Type	SMTP Codes	Can Retry	Action
Soft bounce	421, 450, 451, 452	Yes	Retry automatically, monitor if persistent
Hard bounce	550, 551, 552, 553, 554	Usually no	Remove from list after repeated failures

Soft bounces are temporary. The email system should retry them automatically. Hard bounces indicate a permanent problem. Repeated hard bounces to the same address should trigger removal from your active list.

SMTP Error Categories by Root Cause

Category	Most Common Codes	Typical Fix
Authentication	530, 535, 550	Fix SPF, DKIM, DMARC records
Recipient issues	450, 550, 553	Verify addresses, remove invalid ones
Server problems	421, 451	Wait and retry, check provider status
Mailbox limits	452, 552	Reduce attachments, notify recipient
Spam filtering	550, 554	Improve engagement, reduce complaints
Policy restrictions	550, 554	Review provider terms, adjust practices
Rate limiting	421, 554	Reduce sending speed, warm up IP

How to Fix Most SMTP Errors

Most delivery issues come back to a handful of root causes.

Verify SPF, DKIM, and DMARC are correctly configured and passing
Check your sender reputation using Google Postmaster Tools or Microsoft SNDS
Remove invalid email addresses after repeated hard bounces
Avoid sudden spikes in sending volume — warm up gradually
Keep complaint rates below 0.1% of emails sent
Monitor bounce reports after every campaign
Use a reputable email service provider with established relationships
Check blacklist status on major blocklists
Ensure reverse DNS (PTR record) is configured for your sending IP
Retry temporary (4xx) failures only — do not retry permanent (5xx) failures

Final Thoughts

SMTP errors are designed to help diagnose delivery problems, not confuse senders. While hundreds of SMTP response codes exist, the majority of rejected emails are caused by authentication failures, poor sender reputation, invalid recipients, or temporary server issues.

Understanding what each response code means allows you to fix issues faster, improve deliverability, and reduce bounce rates over time. If you regularly send marketing or transactional emails, learning the difference between temporary (4xx) and permanent (5xx) SMTP errors will save hours of troubleshooting.

Every bounce message carries information. The error code tells you the category of the problem. The text that follows tells you the specific cause. Read both before deciding what action to take.