Emails are bouncing back from comcast.net

Question

Emails are bouncing back unread from comcast.net

Answer 1

When messages to comcast.net bounce back unread, the Comcast mail system is rejecting them. The bounce (NDR) message normally contains an SMTP error code that explains why. Common causes in the provided context are:

1. Invalid or misconfigured recipient address/domain
   • Codes like 5.1.1 Bad Destination Mailbox Address, 5.1.2 Bad Destination System Address, 5.1.3 Bad Destination Mailbox Address Syntax, 5.4.1 Recipient Address Rejected, 5.4.4 Unable to Route, or 5.4.310 DNS Domain Doesn't Exist indicate an issue with the recipient address or domain.
   • Action: Verify the exact @comcast.net address with the recipient (no typos, extra spaces, or invalid characters). If the bounce mentions domain/DNS issues, the problem is on the recipient side and must be fixed by Comcast or the recipient’s admin.
2. Recipient mailbox problems
   • Codes like 550 Mailbox Unavailable, 552 Exceeded Storage Allocation, 5.2.1 Destination Mailbox Disabled, 5.2.2 Destination Mailbox Full, or 5.7.13 User Account Disabled indicate the mailbox is full, disabled, or otherwise unavailable.
   • Action: Ask the recipient to check their mailbox status (full/disabled) with Comcast support and free space or re-enable the mailbox.
3. Policy or authentication issues
   • Codes like 5.7.1 Delivery Not Authorized, Message Refused, 5.7.23 SPF Validation Failed, 5.7.24 SPF Validation Error, 5.7.25 Reverse DNS Validation Failed, or 5.7.26 Multiple Authentication Checks Failed indicate Comcast is rejecting messages based on security or reputation checks.
   • If sending high volume from a custom domain to Microsoft consumer services, similar rules apply as described for error 550 5.7.515: the domain must have valid SPF, DKIM, and DMARC and pass DMARC alignment.
   • Action:
     1. Open the bounce message and review the SMTP status code and text.
     2. If the code is in the 5.7.x range, have the domain’s DNS administrator:
        • Publish and validate SPF and DKIM records for the sending domain.
        • Publish a DMARC record (for example: v=DMARC1; p=none).
        • Ensure the domain in the return-path (5321.MailFrom) aligns with the domain in the From (5322.From) address so DMARC can pass.
4. Temporary issues (soft bounces)
   • Codes like 552 Exceeded Storage Allocation, 554 Transaction Failed, 5.2.2 Destination Mailbox Full, or 5.2.121/5.2.122 Recipient Per Hour Receive Limit Exceeded indicate temporary failures.
   • Action: If the code is 4xx or one of the soft-bounce 5xx codes above, wait and try again later, and ask the recipient to check mailbox limits.

Practical steps:

1. Open one of the bounce (NDR) messages in Outlook/Outlook.com and locate the SMTP error code (e.g., 550, 5.1.1, 5.7.23) and description.
2. Match the code to the explanations above to determine whether the issue is:
   • A bad comcast.net address,
   • A full/disabled Comcast mailbox,
   • A policy/authentication block against the sending domain.
3. If it’s an address/mailbox issue, the Comcast user or Comcast support must fix it.
4. If it’s an authentication/policy issue, the sending domain’s admin must correct SPF/DKIM/DMARC and DNS, then resend.

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References:

• Understand sender reputation in Azure Communication Services email
• Fix NDR error "550 5.7.515" in Outlook.com