SMTP and ESMTP Error Code List

When an email fails to deliver, the receiving server sends a bounce message (also called a bounceback or delivery status notification) back to the sender. These contain SMTP error codes that explain why delivery failed.

SMTP Code Classes Explained

• 2xx codes: Success (you usually won’t see these in bounces).
• 3xx codes: Intermediate (e.g., “start mail input”).
• 4xx codes: Temporary / Soft bounce. The server will usually retry automatically. Common with greylisting, temporary server load, or network issues.
• 5xx codes: Permanent / Hard bounce. Delivery will not be retried. You must fix the underlying issue (bad address, blocked, policy violation, etc.).

Extended SMTP (ESMTP) codes appear in the format X.Y.Z (e.g., 5.1.1):

• First digit = Class (2 = success, 4 = temporary, 5 = permanent)
• Second digit = Subject category
• Third digit = Specific detail

Many bounces show both, for example: 550 5.1.1 User unknown

Most Common SMTP Errors and How to Resolve Them on InMotion Hosting

SMTP Error 550 – Requested Action Not Taken: Mailbox Unavailable / Invalid Address

Also appears as:

• 550 No such user here
• 550 User unknown
• 550 Recipient does not exist
• 550 invalid address
• 550 relaying denied
• 5.1.1 Bad destination mailbox address

What It Means:

This is a permanent (5xx) error. The receiving mail server does not recognize the recipient address or will not accept mail for it.

Common Causes:

• Typo in the recipient’s email address.
• The recipient’s email account was deleted or never existed.
• Recent DNS/MX record changes (domain moved or MX updated).
• The recipient’s server has strict policies or the address is on a suppression list.

How to Fix (If You Are the Sender):

1. Carefully check the recipient address for typos.
2. Contact the recipient through another method (phone, alternate email) and confirm their current email address.
3. Ask them to check if the account still exists on their end.

How to Fix (If Others Are Bouncing When Sending to You):

1. Log into cPanel → Email Accounts and confirm the address exists and is active.
2. Check MX records in cPanel → Zone Editor or MX Entry to ensure they point to InMotion’s mail servers.
3. If you recently transferred the domain or changed nameservers/MX, allow 24–48 hours for propagation and re-test.
4. Create the missing email account if needed.

If the problem persists for legitimate addresses, submit a support ticket with the full bounce message.

SMTP Error 535 – Authentication Required / Incorrect Authentication

Common Variations:

• 535 Incorrect authentication data
• 535 5.7.8 Authentication failed
• multiple servers using same IP requiring auth

What It Means:

The sending server requires authentication that was not provided or was incorrect. On InMotion VPS/Dedicated servers, repeated 535 errors in Exim logs often indicate brute-force login attempts by spammers.

For Shared Hosting Customers:

• Ensure “My outgoing server requires authentication” is enabled in your email client.
• Use the full email address and correct password.
• If you recently changed your email password, update it in all email clients and apps (including phones).

For VPS or Dedicated Server Customers (Advanced):

You can investigate failed login attempts in the Exim mail log:

grep "535 Incorrect" /var/log/exim_mainlog | awk -F"set_id=" '{print $2}' | sort | uniq -c | sort -n

Identify offending IPs and block them at the firewall (e.g., using firewalld, ufw, or your preferred tool). Contact InMotion support if you need assistance interpreting logs or implementing rate limiting/Fail2Ban.

SMTP Error 421 – Service Not Available / Temporary Failure

Common Variations:

• 421 Service not available
• 421 Too many connections
• 421 Greylisting enabled
• please try later
• 4.7.1
• 4.4.5

What It Means:

This is a temporary (4xx) error. The receiving server is currently unable or unwilling to accept the message (busy, greylisting, connection limits, temporary resource issue). Delivery is usually retried automatically.

How to Fix:

• In most cases, do nothing as the sending server will retry.
• If you see many 421 errors in your mail logs or repeated soft bounces:
• Check for high outbound email volume (newsletters, marketing).
• Review your sending practices (avoid spammy content, ensure proper authentication/SPF/DKIM).
• Contact the recipient’s postmaster if the issue persists for important mail.
• For InMotion customers seeing widespread 421s when sending: Submit a ticket so our team can review outbound delivery health.

Other Common 5xx Permanent Errors

552 – Requested mail action aborted: exceeded storage allocation
Mailbox is full on the recipient’s end. Ask the recipient to delete old messages or increase quota. If this is your own mailbox, check cPanel → Email Accounts → Quota or contact support.

553 – Requested action not taken: mailbox name not allowed
Often caused by invalid characters in the local part of the address or policy restrictions on the receiving server. Verify the address format.

554 – Transaction failed
Generic permanent failure. Could be content filtering (spam/virus), policy violation, or DNS issues. Review the full bounce message for more clues. Test with a simple plain-text email.

450 – Requested mail action not taken: mailbox unavailable
Temporary (4xx) version of mailbox issues. Usually retries. Can also indicate greylisting or the recipient server is temporarily rejecting.

Full SMTP Error Code Reference

200–300 Series (Informational / Success)

• 211 – System status / help reply
• 214 – Help message
• 220 – Service ready
• 221 – Service closing transmission channel
• 250 – Requested mail action completed, OK
• 354 – Start mail input; end with <CRLF>.<CRLF>

400 Series (Temporary Failures – Usually Retried)

• 421 – Service not available
• 450 – Mailbox unavailable (temporary)
• 451 – Local error in processing; try again later
• 452 – Insufficient system storage
• 454 – TLS not available (temporary)

500 Series (Permanent Failures – Action Required)

• 500 – Syntax error, command unrecognized
• 501 – Syntax error in parameters
• 502 – Command not implemented
• 503 – Bad sequence of commands
• 504 – Command parameter not implemented
• 510 / 512 – Bad recipient address or domain not found
• 530 – Access denied / Authentication required
• 535 – Authentication failed
• 550 – Mailbox unavailable / Invalid address
• 551 – User not local; try forward path
• 552 – Exceeded storage allocation (mailbox full)
• 553 – Mailbox name not allowed
• 554 – Transaction failed

Extended SMTP (ESMTP) Status Codes (x.y.z)

These provide more granular detail:

x.1.x – Address Status

• X.1.1 – Bad destination mailbox address (very common with 550)
• X.1.3 – Bad destination mailbox address syntax
• X.1.7 – Bad sender’s mailbox address syntax

x.2.x – Mailbox Status

• X.2.1 – Mailbox disabled, not accepting messages
• X.2.2 – Mailbox full
• X.2.3 – Message length exceeds limit

x.3.x – Mail System Status

• X.3.1 – Mail system full
• X.3.4 – Message too big for system

x.4.x – Network/Routing

• X.4.1 – No answer from host
• X.4.2 – Bad connection
• X.4.7 – Delivery time expired (message too old)

x.5.x – Protocol

• X.5.1 – Invalid command
• X.5.2 – Syntax error
• X.5.3 – Too many recipients

x.7.x – Security/Policy

• X.7.1 – Delivery not authorized, message refused (spam/policy block)
• X.7.2 – Mailing list expansion prohibited