Email Bounce back

Question

I'm having issues over the last few days with people trying to send me emails with attachements and them getting bounce back notifications. Some of their other emails are taking 3 days to arrive. I've checked my storage on my one drive and I have 1TB available so it's not full. Please can someone help as these are important work emails.

Answer 1

Hi Louise Burton,

Thank you for posting your question in the Microsoft Q&A forum.

To better understand the situation and provide more targeted assistance, could you please share some additional details?

1. What is the exact error/NDR message that senders are receiving? Is it consistent across all senders, or does each sender get a different bounce-back message?
2. What file types are being attached (e.g., PDF, DOCX, XLSX, ZIP)? What are the approximate file sizes?
3. Have you noticed any cases where emails with attachments are still getting through successfully? If so, is there any difference in sender, file type, or file size?
4. If you try sending an email with an attachment back to those same senders, does it go through without issues?

Additionally, since this appears to be a business account, it would be worth reaching out to your IT admin to check whether any recent changes were made to mail flow rules, transport rules, or Microsoft Defender Safe Attachments policies, as these can sometimes cause unexpected bounce-backs or delivery delays without any changes on the user side.

Looking forward to your input.

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