Why Did I Receive a Mail Failure Notice After Sending an Email?
When you send an email in WebMailPRO (or through your connected email client) and receive a Mail Delivery Failure, Bounce-Back, Undeliverable Mail, or Mailer-Daemon notice, it means the receiving mail server could not accept your message.
Bounce-backs can occur for a number of common reasons—most of which are easily fixable.
This guide explains the most frequent causes, how WebMailPRO users can check them, and when to contact K12USA Support.
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Common Reasons You Received a Mail Failure Notice
Below are the most frequent reasons mail cannot be delivered.
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1. The recipient’s email address is misspelled
This is the #1 cause.
Even a small typo — an extra character, wrong domain, or missing period — will cause delivery failure.
Example:
• Correct: jsmith@school.org
• Mistyped: jsmiith@school.org
• Wrong domain: jsmith@shcool.org
How to check:
Review the bounce-back message. It usually lists the exact address attempted.
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2. The recipient’s mailbox is full
This is extremely common with:
• Parents using free email accounts
• Staff with mailbox limits
• Systems that auto-archive poorly
Bounce-backs often say:
“Mailbox full,” “Quota exceeded,” or “User has exceeded storage allocation.”
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3. The recipient’s mail server temporarily rejected the message
Mail servers can reject mail if:
• They are overloaded
• Their spam filters are recalibrating
• They have temporary outages
• The sender is sending too quickly (rate limiting)
Example: Comcast / Xfinity issue
Some domains (like Comcast) temporarily slow down or reject high volumes of mail. This is often not specific to you—it’s their system reacting to bulk messaging.
Bounce-back wording may include:
“Greylisted,” “Rate limited,” “Try again later,” or “Temporary failure.”
Your message may still deliver after the server retries.
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4. The recipient’s mail server blocked the message due to filtering
Some domains use aggressive anti-spam filtering.
Your message may be blocked because:
• It contains certain keywords
• It has an attachment type they disallow
• It has links they consider suspicious
• Your school’s domain is unknown to them
• Their filtering settings are extremely strict
Bounce-back wording may include:
“Message content rejected,” “Blocked,” “Security policy,” or “Prohibited attachment.”
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5. The recipient’s domain is using SPF/DKIM/DMARC and failed validation
If your domain’s authentication checks don’t align with the recipient’s policies, their server may reject your mail.
This sometimes occurs when:
• You send mail through both WebMailPRO and another service
• You have forwarding rules
• A third-party system sends mail on your behalf
Bounce-back wording may include:
“SPF fail,” “DKIM fail,” “DMARC fail,” or “Message not aligned.”
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6. The receiving mail server blocked our sending IP
This is not common, but can occur for legitimate reasons, such as:
• A school accidentally sending bulk/bounce floods
• A compromised user account sending out many messages
• A district sending large notification batches
• Incorrect configuration on the recipient’s side
Bounce-back wording may include:
“Blocked,” “RBL listing,” “Blacklisted,” or “IP reputation.”
If this happens, we can help resolve it.
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7. The receiving domain does not exist or has DNS errors
If the domain is:
• Newly created
• Misconfigured
• Expired
• Temporarily offline
…the message cannot be delivered.
Bounce wording may include:
“No such domain,” “Host not found,” or “NXDOMAIN.”
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How WebMailPRO Users Can Check the Cause
1. Read the bounce-back message carefully
Bounce messages usually include:
• The recipient address attempted
• The time/date
• The reason (in plain text or technical code)
• The receiving server’s hostname
• A numeric SMTP status code (like 550, 551, 452, etc.)
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Common SMTP Codes and What They Mean
Code Meaning Typical Cause
550 Permanent failure Bad address, blocked, policy rejection
551 User not local Address does not exist
552 Quota exceeded Recipient mailbox full
421 / 451 / 452 Temporary failure Rate limiting, greylisting, busy server
554 Transaction failed Hard block / filter rejected
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2. Verify the email address spelling
Double-check that:
• The username is correct
• The domain is correct
• There are no extra spaces or characters
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3. Try sending a simple test message
Use a normal text-only email with no attachments.
If it sends successfully, the issue may be related to:
• Attachments
• Formatting
• Templates
• A previous message’s content
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Need Help? Forward the Bounce to Us!
If you’re unsure what the bounce means—or the issue keeps happening—we’re happy to help.
Please forward the full bounce-back email to:
support@k12usa.com
Having the original bounce helps us see:
• The exact SMTP error
• Which server rejected the mail
• Whether our sending IP is blocked
• What filtering rule was triggered
Important:
Please include any additional information such as:
• What address you were sending to
• Whether this has worked before
• Whether you are using WebMailPRO or another client (Outlook, iOS Mail, etc.)
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We’re Here to Help
Our team can quickly analyze bounce logs and determine why your message failed. Many issues can be resolved within minutes.
If you need assistance:
support@k12usa.com
877-225-0100