Letter return identified as SPAM

When outgoing messages are repeatedly identified as spam, the issue is typically with how the sending account or message content is being classified by the mail system, not with the specific file type (text vs PDF attachment).

For Outlook.com / Outlook on the web, use these steps and checks:

1. Verify that the account is not blocked for outbound spam
   • If a large portion of recent mail was classified as spam, the service can temporarily block sending or route messages through a high‑risk pool, which increases the chance of spam classification.
   • Check for any non‑delivery reports (NDRs) or error codes in the bounce messages. If the NDR indicates the sending IP or account is blocked, follow the instructions in the NDR. For example, some NDRs direct the sender to a delist portal such as https://sender.office.com/ to request removal from a blocked senders list.
2. Review message content and URLs
   • Microsoft 365 uses URL filtering and marks messages as spam if they contain URLs associated with known spam or malicious content.
   • If the email or PDF contains links, remove or change any suspicious or shortened URLs and resend.
3. Reduce spam‑like characteristics
   • Avoid sending the same message repeatedly in a short period, especially to many recipients.
   • Avoid spam‑style wording (overly promotional language, misleading subjects, or content that resembles known spam patterns).
4. Check for malware or malicious indicators
   • Ensure the PDF and any other attachments are clean (scan with up‑to‑date antivirus). Messages with malware indicators are more likely to be blocked or treated as spam.
5. Work with the recipient
   • Ask the recipient (or their admin, if they use Microsoft 365) to:
     • Check their Junk Email folder.
     • Use message trace or security reports on their side to see why the message was marked as spam.
     • Submit the message as a false positive to Microsoft for analysis using the built‑in reporting options. This helps improve future deliverability.
6. If you receive a specific NDR code
   • Use the error code in the NDR to look up the cause in the Microsoft 365 NDR documentation referenced in the mail‑flow troubleshooting guidance, and follow the recommended remediation steps.

If the problem persists even after cleaning content and attachments and following NDR instructions, contact Outlook.com/consumer support so they can check whether the account or sending IP has been incorrectly classified as a spam source.

References:

• External senders - Troubleshoot email sent to Microsoft 365
• Frequently asked questions - Anti-spam protection
• I receive a 550, 553, or relay-prohibited error when sending email messages