Why Did I Receive a Mail Failure Notice After Sending an Email?
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Overview
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When you send an email using WebMailPRO (or through a 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.

Mail failure notices 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 identify them,
and when to contact K12USA Support for assistance.

Common Reasons You Received a Mail Failure Notice
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Below are the most frequent reasons email messages cannot be delivered.

1. The recipient’s email address is misspelled
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This is the #1 cause of mail delivery failures.

Even a small typo—an extra character, wrong domain, or missing period—will cause delivery to fail.

Examples:

• 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.

2. The recipient’s mailbox is full
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This is extremely common with:

• Parents using free email accounts
• Staff with mailbox size limits
• Systems that do not auto-archive properly

Bounce-backs often include messages such as
“Mailbox full,” “Quota exceeded,” or
“User has exceeded storage allocation.”

3. The recipient’s mail server temporarily rejected the message
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Mail servers may temporarily reject messages if:

• The server is overloaded
• Spam filters are recalibrating
• There is a temporary outage
• The sender is rate-limited

Example: Some providers (such as Comcast/Xfinity) may temporarily slow or reject
high volumes of mail. This is usually a system-side response, not a permanent block.

Bounce-backs may include wording such as
“Greylisted,” “Rate limited,” “Try again later,” or
“Temporary failure.”

4. The recipient’s mail server blocked the message due to filtering
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Some email domains use aggressive anti-spam filtering. Messages may be blocked if they:

• Contain certain keywords
• Include a disallowed attachment type
• Contain links flagged as suspicious
• Originate from an unfamiliar school domain
• Trigger strict filtering policies

Bounce wording may include
“Message content rejected,” “Blocked,” “Security policy,” or
“Prohibited attachment.”

5. SPF, DKIM, or DMARC authentication failed
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If your domain’s email authentication does not align with the recipient’s policies,
their server may reject the message.

This may occur when:

• Email is sent through multiple services
• Forwarding rules are in place
• A third-party system sends mail on your behalf

Bounce-backs may reference
“SPF fail,” “DKIM fail,” “DMARC fail,” or
“Message not aligned.”

6. The receiving mail server blocked the sending IP
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This is uncommon but can occur due to:

• Bulk or automated mail activity
• A compromised user account
• Large notification or alert batches
• Misconfiguration on the recipient’s side

Bounce-backs may include
“Blocked,” “RBL listing,” “Blacklisted,” or
“IP reputation.”

7. The receiving domain does not exist or has DNS errors
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If a domain is new, expired, misconfigured, or temporarily offline, mail cannot be delivered.

Bounce wording may include
“No such domain,” “Host not found,” or
“NXDOMAIN.”

Common SMTP Codes and What They Mean
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Need Help? Forward the Bounce to Us
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If you’re unsure what the bounce message means—or the issue keeps happening—we’re happy to help.
Please forward the full bounce-back email to:

📧 support@k12usa.com

Including the full bounce helps us determine:

• The exact SMTP error
• Which server rejected the message
• Whether a sending IP is blocked
• What filtering rule was triggered

Please also include:

• The recipient address
• Whether this has worked before
• Which email client you are using (WebMailPRO, Outlook, mobile, etc.)