Why do I suddenly get a lot of emails sent back to my contacts because they say they are unwanted messages?

Question

Today I started sending emails to my students, as always, with exercises attached, and most of them have been returned. The outlook system tells me that they have been returned because it is suspected that the content is unwanted (spam). However, the content I attach is the same type as always

Answer 1

Dear @Morrison Faye,

I understand how frustrating it is when your legitimate class materials are suddenly flagged as spam, especially when your routine hasn't changed.

This issue typically occurs because automated email protection filters (such as Exchange Online Protection) evaluate bulk mailings, attachment characteristics, and sender reputation dynamically. If several outbound emails trigger a spam signal simultaneously, system filters automatically return them to prevent account compromise.

Here are the primary causes and steps to resolve this:

Step 1: Check for Anti-Spam Outbound Block Lists

If a high volume of returned emails occurred quickly, your account or sending IP may have been temporarily restricted from sending outbound mail.

• Ask your Microsoft 365 Global Administrator to check the Restricted entities page in the Microsoft Defender portal. If your address appears there, the admin can manually unblock your account.

Step 2: Optimize the Attachment & File Delivery

Attaching identical files (especially .zip, macro-enabled docs, or large PDFs) to dozens of recipients at once frequently triggers spam heuristic rules. Instead of attaching files directly, upload the exercises to your OneDrive for Business or SharePoint, then share a direct link in your email. This completely bypasses attachment spam filters.

Step 3: Adjust Sending Volume & Recipient Formatting

Sending an identical email to many recipients placed in the To: or Cc: field flags bulk-spam rules.

• Put student addresses in the Bcc: line or send emails in smaller batches. If you are emailing large classes regularly, consider using Microsoft Teams for assignment distribution instead.

Step 4: Verify Email Authentication (For Admins)

If you are using a custom domain (e.g., @your school.edu), spam filters on the students' receiving email servers may reject messages if authentication policies are missing.

Ensure your organization has valid SPF, DKIM, and DMARC records configured in DNS.

For detailed steps on handling outbound spam blocks and troubleshooting deliverability, please refer to the official documentation:

Remove blocked users from the Restricted entities page

Email authentication in Exchange Online Protection

Please start by uploading the exercises to OneDrive and sharing the link to get your emails flowing again today. Let me know if you need further assistance.

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Answer 2

This usually happens because the message is being classified as spam by mail filters, even if the attachment type is the same as before. A sudden change can be caused by one of these common reasons:

• Message content triggers: the body, subject, or attachments now match spam-like patterns such as excessive links, URL shorteners, embedded scripts, image-only content, or certain HTML content.
• Sender reputation: the sending account, domain, or sending IP may now have a poorer reputation.
• Email authentication failure: if SPF, DKIM, or DMARC checks fail, messages are treated with higher suspicion.
• Recipient-side blocking: some recipient systems intentionally block messages they believe are spam.

For a personal Outlook.com-style scenario, the most practical steps are:

1. Send a simple test message
   Send one message with plain text only, with no attachment, no links, no images, and no signature. If that works, the problem is likely the attachment or message formatting.
2. Add content back gradually
   First resend with the text only. Then add the attachment. Then add any links or formatting. This helps identify what triggers the spam detection.
3. Review the attachment and message format
   Attachments plus HTML-formatted email are more likely to be scrutinized. Keep the message short and direct, and avoid unnecessary formatting.
4. Check the delivery failure notice
   In the returned message, look for the diagnostic section. It usually explains why the message was rejected.
5. If messages are being blocked by the recipient system
   If the error mentions terms like blocked, listed in, spamcop, dynablock, blackhole, or spamhaus, the recipient’s email provider is blocking the message source. In that case, the recipient’s provider support team needs to remove the block.
6. If using a custom domain
   Verify that SPF, DKIM, and DMARC are correctly configured. Authentication failures are a common reason legitimate mail is flagged as spam.

If the issue is only with messages that include exercises as attachments, the most likely cause is that the message content or attachment is now matching updated spam-filter patterns rather than a change in the recipients or attachment type itself.

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