I can't send any emails containing at attachment. How can I fix this?

Anonymous
2025-06-25T16:16:46+00:00

I'm not able to send any emails containing an attachment. They immediately bounce back saying: your message wasn't delivered because the recipient's email provider rejected it.

Diagnostic information for administrators:

Generating server: CH3PR20MB6711.namprd20.prod.outlook.com
Remote server returned '550 5.7.520 Message blocked because it contains content identified as spam. AS(4810)'

This problem is happening for 1 of my 2 Outlook accounts (one will send with attachment, the other won't)

I have tried sending to different email addresses and get the same result, so I don't think the recipient has anything to do with it.

I am able to send emails without at attachment.

The attachment is 34 KB, so not too large.

I have read about checking settings in the "Trust Center" but can't figure out how/where to find that. I did look at all the settings (by clicking on the gear icon in the top right corner) but didn't find the Trust Center or anything related to this problem.

I have tried shutting everything down and rebooting, and the problem still remains.

Advice needed!

Outlook | Windows | New Outlook for Windows | For business

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EmilyS726 264.4K Reputation points Independent Advisor
2025-06-25T17:30:41+00:00

Yeah, I am seeing more and more users reporting this issue. I do believe it has to do with updated security changes made from Microsoft.

You can use this instruction to view rich text vs. plain text: https://support.microsoft.com/en-us/office/chan...

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EmilyS726 264.4K Reputation points Independent Advisor
2025-06-25T16:58:31+00:00

Hello,

Any chance you are trying to send an email with a PDF attachment?

More and more email services providers have updated their spam filters due to rising zero-day phishing PDF campaigns. PDFs + HTML email are commonly used in phishing, so security systems scrutinize these more.

You can consider these alternatives if you need to send PDF attachment -

  1. Share it via a OneDrive link instead for the recipient to download.
  2. Zip it into a .zip file and attach it.
  3. If your email format is rich text/html, convert it to plain text. - I think this might be why one of them works as it might be in plain text.

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  1. EmilyS726 264.4K Reputation points Independent Advisor
    2025-06-25T19:17:57+00:00

    You are welcome. Glad you sorted this.

    If all is good, feel free to mark the answer for this thread, or let me know if you need anything else from me.

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  2. Anonymous
    2025-06-25T19:16:46+00:00

    Thank you so SO much for your help! I first tried sending my PDF in a zip file, and it worked. You were also right about the formatting... email was set to HTML while the other one was set to plain text. Changing that made all the difference. Much appreciated!!! =)

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  3. Anonymous
    2025-06-25T17:25:52+00:00

    Thank you for the reply. Yes, it's PDF files I'm sending. Has always worked fine until yesterday. Can you tell me how/where to check which format my email is in?

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