Email Bounce Reasons Guide

Overview

An email bounce happens when a mail server cannot deliver a message to the recipient. When this occurs, the sender usually receives an automatic message called a:

• Bounce Message
• Delivery Failure
• Delivery Status Notification — DSN
• Non-Delivery Report — NDR
• Mail Delivery Failed
• Undelivered Mail Returned to Sender

The bounce message usually includes the reason for the failure, such as an incorrect email address, a full mailbox, blocked message, DNS issue, authentication failure, or spam-related rejection.

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Types of Email Bounces

1. Soft Bounce

A soft bounce is a temporary delivery failure. The sending mail server may retry delivery automatically.

Common causes include:

• The recipient mailbox is full.
• The recipient mail server is temporarily unavailable.
• The recipient server is overloaded.
• The message is too large.
• The recipient server uses greylisting.
• There is a temporary DNS or network issue.

Soft bounces usually appear with 4xx status codes, such as:

• 421
• 450
• 451
• 452
• 4.2.2
• 4.4.1

Recommended action: wait and try again later. If the issue continues, contact the recipient or your hosting support team depending on the error.

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2. Hard Bounce

A hard bounce is a permanent delivery failure. The message will not be delivered unless the root cause is fixed.

Common causes include:

• The recipient address does not exist.
• The recipient domain does not exist.
• The message was rejected by the recipient’s security policy.
• The sending server IP address is blocked.
• SPF, DKIM, or DMARC authentication failed.
• The recipient has blocked the sender.
• The receiving server does not accept mail from the sender’s server.

Hard bounces usually appear with 5xx status codes, such as:

• 550
• 551
• 552
• 553
• 554
• 5.1.1
• 5.2.2
• 5.7.1

Recommended action: read the bounce reason, correct the address or email settings, then contact support if the issue is related to the server, DNS, email authentication, or mail reputation.

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How to Read a Bounce Message

When you receive a bounce message, look for the following details.

1. The affected email address

Example:

[email protected]

This is the address that did not receive the message.

2. The error code

Examples:

550 5.1.1

554 5.7.1

451 4.7.1

3. The error text

Examples:

User unknown

Mailbox full

Message rejected due to spam policy

SPF check failed

4. The server that rejected the message

The rejection may come from:

• The recipient mail server.
• The sender mail server.
• An external filtering service.
• A security gateway such as Google Workspace, Microsoft 365, Proofpoint, or Mimecast.

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Common Bounce Reasons and What They Mean

Error code or message	Meaning	Likely side of the issue	Recommended action
550 5.1.1	Recipient mailbox does not exist	Recipient side	Check the email address and ask the recipient to confirm it
550 No such user	User does not exist on the recipient server	Recipient side	Confirm the correct email address
553 Invalid recipient	Recipient address is invalid	Sender or recipient side	Check the email format and spelling
552 Mailbox full or 5.2.2	Recipient mailbox is full	Recipient side	Ask the recipient to free up mailbox space
552 Message size exceeds limit	Message or attachment is too large	Sender or recipient side	Reduce attachment size or use a download link
554 5.7.1 Message rejected	Message rejected by a security or spam policy	Usually recipient side, but may be related to sender reputation	Review message content and SPF/DKIM/DMARC settings
550 SPF failed	SPF authentication failed	Sender side or DNS side	Check the domain’s SPF record
DKIM signature failed	DKIM validation failed	Sender side or DNS side	Check that DKIM is enabled and correctly published
DMARC policy reject	DMARC policy rejected the message	Sender side	Check SPF, DKIM, and DMARC alignment
421 Service not available	Recipient server is temporarily unavailable	Usually recipient side	Try again later
451 Temporary local problem	Temporary mail server problem	Sender or recipient side	Wait and retry; contact support if it continues
450 Mailbox unavailable	Mailbox temporarily unavailable	Usually recipient side	Try again later
554 IP blocked	Sending server IP address is blocked	Sender side or server reputation issue	Contact support to check IP reputation and blocklists
Relay access denied	The mail server does not allow relaying	Sender side	Check SMTP settings and authentication
Authentication required	SMTP authentication is required	Sender side	Enable SMTP authentication in the mail client
Connection timed out	Connection to mail server timed out	Network, DNS, or recipient server issue	Try again later and check connectivity
DNS lookup failed	DNS or MX lookup failed	Usually recipient DNS or domain issue	Check the recipient domain’s MX records

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When the Issue Is Usually on the Sender’s Side

The issue is usually related to the sender, sender domain, hosting account, or mail server in the following cases.

1. SPF, DKIM, or DMARC failure

Example errors:

• SPF check failed
• DKIM signature failed
• DMARC policy reject
• Unauthenticated email from domain is not accepted

Possible causes:

• DNS records are missing or incorrect.
• The domain does not have a valid SPF record.
• DKIM is not enabled.
• DMARC policy is strict but SPF/DKIM are not configured correctly.
• Mail is being sent from a server that is not authorized to send for the domain.

What to do:

• Check Email Deliverability in cPanel.
• Make sure SPF and DKIM records are valid.
• Make sure mail is sent from an authorized mail server.
• If using an external mail service such as Google Workspace, Microsoft 365, Mailchimp, or another SMTP provider, include that service in the SPF record.

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2. Incorrect SMTP settings

Example errors:

• Authentication required
• Relay access denied
• Invalid login
• SMTP authentication failed

Possible causes:

• Incorrect username or password.
• Wrong SMTP server.
• Wrong SMTP port.
• SMTP authentication is disabled.
• SSL/TLS settings are incorrect.

What to do:

• Check the mail client settings.
• Use the full email address as the username, for example: [email protected].
• Use the correct SMTP server from the hosting control panel.
• Use port 465 with SSL or port 587 with TLS.
• Confirm that the mailbox password is correct.

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3. Poor server or IP reputation

Example errors:

• IP blocked
• Blacklisted
• Poor reputation
• Rejected due to spam source
• Your IP is listed in...

Possible causes:

• A mailbox is compromised and sending spam.
• A website script is sending unwanted email.
• A large number of emails were sent in a short time.
• The server uses a shared IP address affected by another sender.
• The sending IP is listed on an external blocklist.

What to do:

• Change passwords for suspicious mailboxes.
• Scan the website for malware.
• Stop unauthorized bulk sending.
• Review recent sending activity.
• Contact support to check server reputation and blocklists.

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4. Sending limit exceeded

Example errors:

• Rate limit exceeded
• Too many messages
• Daily sending quota exceeded
• Too many recipients

Possible causes:

• Too many emails were sent in a short period.
• The message was sent to too many recipients.
• A regular hosting mailbox is being used for newsletters or marketing campaigns.
• A compromised account is sending large volumes of mail.

What to do:

• Reduce the number of messages sent at one time.
• Avoid sending to large mailing lists from a normal hosting mailbox.
• Use a dedicated email marketing platform for campaigns.
• Contact support to confirm the sending limits for your service.

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5. Message content rejected as spam

Example errors:

• Message rejected as spam
• Content rejected
• Spam detected
• Blocked due to content

Possible causes:

• The message contains suspicious links.
• The message uses aggressive marketing language.
• The message includes blocked attachment types.
• The message uses URL shorteners.
• The signature contains too many links or images.
• The same message was sent to many recipients.

What to do:

• Send a plain text test message without links or attachments.
• Remove shortened links.
• Reduce images and links in the signature.
• Avoid suspicious attachments such as .exe files or untrusted compressed files.
• Send to one recipient first as a test.

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When the Issue Is Usually on the Recipient’s Side

The issue is usually related to the recipient mailbox, recipient domain, or recipient mail server in the following cases.

1. The recipient address is incorrect or does not exist

Example errors:

• User unknown
• No such user
• Recipient address rejected
• Invalid recipient
• Mailbox unavailable

What to do:

• Check the spelling of the email address.
• Make sure there are no spaces before or after the address.
• Confirm the correct domain.
• Ask the recipient to send you their correct address.
• Try sending from another email account for comparison.

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2. The recipient mailbox is full

Example errors:

• Mailbox full
• Quota exceeded
• Over quota
• Insufficient storage

What to do:

• Ask the recipient to delete old emails.
• Ask the recipient to empty Trash and Spam folders.
• If the recipient mailbox is under your hosting account, increase the mailbox quota or upgrade the plan.

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3. The recipient server rejected the message

Example errors:

• Message rejected by recipient server
• Recipient server rejected the message
• Blocked by policy
• Access denied

Possible causes:

• The recipient uses strict email filtering.
• The sender address or domain is not trusted by the recipient server.
• The recipient has blocked the sender.
• The recipient’s mail provider does not accept certain content or attachments.

What to do:

• Ask the recipient to check Spam or Junk.
• Ask the recipient to add your email address to their safe sender list.
• Resend the message without attachments or links.
• If the issue continues, the recipient should contact their mail provider.

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4. The recipient server is temporarily unavailable

Example errors:

• Connection timed out
• Server temporarily unavailable
• 421 Service not available
• Temporary failure

What to do:

• Wait and try again later.
• Try sending to another recipient.
• If the issue happens only with one recipient domain, the issue is likely on the recipient side.
• If the issue happens with all recipient domains, check the sender mailbox, domain settings, or hosting server.

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When the Issue May Be from the Hosting Provider

The issue may need hosting-provider investigation in the following cases:

• All outgoing messages fail from more than one mailbox.
• All users under the same domain experience the same issue.
• cPanel Track Delivery shows a failure before the message is sent externally.
• The mail server cannot connect to external recipient servers.
• Many messages bounce because of IP blocklist or reputation issues.
• Email Deliverability shows SPF/DKIM issues that the customer cannot resolve.
• The mail service is not working.
• DNS or MX records hosted with the provider appear incorrect.
• Mail cannot be sent from both Webmail and mail clients.
• There are no delivery logs visible even though messages were sent.

In these cases, contact support and provide the full bounce message.

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Customer Troubleshooting Steps Before Contacting Support

Step 1: Read the full bounce message

Do not rely only on the email subject. Open the bounce message and look for:

• The error code.
• The affected recipient address.
• The exact error text.
• The server that rejected the message.
• The time of the failed delivery.

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Step 2: Check whether the issue affects one recipient or all recipients

Ask:

• Does the issue happen with one email address only?
• Does the issue happen with one domain only, such as Gmail, Outlook, or a company domain?
• Does the issue happen with all outgoing emails?
• Are all mailboxes under the domain affected?
• Does incoming mail work but outgoing mail fails?
• Does outgoing mail work but incoming mail fails?

If the issue affects one recipient only, it is usually related to the recipient, recipient address, or recipient server.

If the issue affects all recipients, it is usually related to the sender mailbox, sender domain, mail server, DNS, or authentication settings.

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Step 3: Check the email address

Make sure:

• The email address is spelled correctly.
• There are no extra spaces.
• The domain is correct.
• The recipient mailbox still exists.

Example of an incorrect address:

[email protected]

Correct address:

[email protected]

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Step 4: Send a simple test message

Send a plain message with text only. Do not include:

• Attachments.
• Links.
• Images.
• Long signatures.
• Multiple recipients.

If the simple message is delivered, the original issue may be related to message content, attachments, links, or recipient filtering.

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Step 5: Check the email client settings

If using Outlook, Apple Mail, Thunderbird, or a phone mail app, check:

• SMTP server.
• Username.
• Password.
• SMTP port.
• SSL/TLS settings.
• SMTP authentication.

Also try sending from Webmail.

If sending works from Webmail but fails from the mail app, the issue is most likely in the mail app settings.

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Step 6: Check Email Deliverability in cPanel

1. Log in to cPanel.
2. Go to Email.
3. Open Email Deliverability.
4. Check the domain status.
5. Repair any SPF or DKIM issues shown.
6. If DNS is managed outside the hosting account, copy the required records and add them at the external DNS provider.

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Step 7: Use Track Delivery in cPanel

1. Log in to cPanel.
2. Go to Email.
3. Open Track Delivery.
4. Search by sender or recipient email address.
5. Review the message status.

Common statuses:

• Delivered: The message was delivered to the receiving server.
• Deferred: Delivery was delayed and the server may retry.
• Failed: Delivery failed.
• Rejected: The message was rejected.
• Accepted: The message was accepted by the server.
• In Progress: The message is still being processed.

Important note:
If the status is Delivered, this usually means the sender’s server handed the message to the recipient server. If the recipient cannot find it, they should check Spam/Junk or contact their mail provider.

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How to Identify the Side of the Issue

The issue is usually on the sender’s side if:

• The error mentions SPF, DKIM, or DMARC.
• The message is rejected because of IP reputation or blocklisting.
• All outgoing messages fail.
• Sending fails from both Webmail and mail clients.
• The error says Authentication required or Relay access denied.
• Sending stops because of sending limits.
• The message contains links, large attachments, or content that triggers spam filters.

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The issue is usually on the recipient’s side if:

• The error says User unknown or No such user.
• The error says Mailbox full or Quota exceeded.
• The issue happens with one recipient only.
• The recipient server rejects the message because of its own policy.
• The recipient server is temporarily unavailable.
• cPanel Track Delivery shows Delivered, but the recipient cannot find the message.

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The issue may require hosting support if:

• Several mailboxes under the same domain cannot send.
• Several users on the same server have the same issue.
• The error points to DNS, mail server, or routing issues.
• The sending IP address is blocked.
• Webmail cannot send either.
• There are no visible delivery attempts after sending.
• Email Deliverability cannot be repaired from the customer side.

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Practical Examples

Example 1: 550 5.1.1 User unknown

Meaning: The recipient mailbox does not exist.

Likely side: Recipient side.

Action:

• Check the address spelling.
• Ask the recipient to confirm the correct address.
• Contact support only if the address is confirmed correct and the issue happens with several recipients.

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Example 2: 552 Mailbox full

Meaning: The recipient mailbox is full.

Likely side: Recipient side.

Action:

• Ask the recipient to free up mailbox space.
• If the mailbox belongs to your hosting account, increase its mailbox quota from cPanel.

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Example 3: 554 5.7.1 Message rejected as spam

Meaning: The message was rejected by an anti-spam policy.

Likely side: Message content, sender reputation, sender authentication, or recipient policy.

Action:

• Send a plain text test message.
• Remove attachments and links.
• Check SPF, DKIM, and DMARC.
• If the issue continues with multiple recipients, contact support.

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Example 4: Relay access denied

Meaning: The server does not allow the message to be sent using the current settings.

Likely side: Sender SMTP settings.

Action:

• Check the SMTP server.
• Enable SMTP authentication.
• Use the full email address as the username.
• Confirm the password and port.

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Example 5: SPF failed

Meaning: The receiving server could not verify that the sending server is authorized to send email for the domain.

Likely side: Sender DNS settings.

Action:

• Check Email Deliverability in cPanel.
• Repair the SPF record.
• If using an external mail provider, add it to SPF.
• Contact support if you are not sure which SPF record is correct.

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When to Contact Support

Contact technical support if:

• You cannot send email to any recipient.
• The issue happens from all mailboxes under the same domain.
• The error mentions SPF, DKIM, or DMARC and you are not sure how to fix it.
• The error mentions IP blocked, blacklist, or poor reputation.
• Webmail cannot send messages.
• Track Delivery shows Failed or Rejected without a clear reason.
• No delivery attempts appear in Track Delivery.
• You suspect a mailbox or website has been compromised.
• Temporary 4xx errors continue for several hours.
• The error appears to come from your hosting server, not the recipient server.
• Mail log investigation is required.

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Information to Include When Opening a Support Ticket

To help support investigate faster, include:

1. Sender email address.
2. Recipient email address.
3. Date and time of sending.
4. Full bounce message.
5. Exact error code.
6. Whether the issue happens with one recipient or all recipients.
7. Whether the message was sent from Webmail, Outlook, phone, or another mail app.
8. Whether the message includes attachments or links.
9. Whether a plain text test message was attempted.
10. Screenshot or result from cPanel Track Delivery, if available.

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Best Practices to Reduce Email Bounces

• Verify recipient addresses before sending.
• Do not send marketing campaigns from regular hosting email accounts.
• Enable SPF, DKIM, and DMARC.
• Avoid shortened links in important messages.
• Avoid very large attachments.
• Use download links instead of large attachments.
• Avoid sending to many recipients at once.
• Monitor bounce messages regularly.
• Remove invalid addresses from mailing lists.
• Use strong passwords for mailboxes.
• Enable two-factor authentication where available.
• Scan websites regularly for malware.
• Do not reply to spam or forward spam messages to many recipients.

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Quick Summary

An email bounce does not always mean there is a hosting issue. The cause may be an incorrect recipient address, full recipient mailbox, recipient security policy, sender DNS configuration, sender reputation, or message content.

The best troubleshooting flow is:

1. Read the exact bounce error.
2. Check whether it is a temporary 4xx or permanent 5xx error.
3. Confirm whether the issue affects one recipient or all recipients.
4. Review cPanel Track Delivery.
5. Check cPanel Email Deliverability.
6. Contact support if the issue is general, server-related, reputation-related, or unclear.