Outlook several message are bouncing back

Dear Graziella, 

Thank you for reaching out here in the Microsoft Q&A forum and for providing the exact error message. I understand how incredibly frustrating and disruptive this must be for your professional work, especially after two months and with a paid account.  

The error 550 5.7.520 with the code AS (4810) is key. It tells us the block is occurring on Microsoft's outbound mail service because our protection systems have flagged the message content. Since this affects multiple recipients, the issue is likely tied to your account's sending reputation or a specific characteristic of your emails. 

First of all, let's quickly rule out common content triggers. Please send a brand new, plain-text email (no attachments, no signature, no previous thread) to a trusted contact who has previously received your emails. 

• On Outlook on the web: Click New message, then select "⋮" (More actions) > "Switch to plain text". 
• On Outlook for Mac: Click New Email, then go to the Options tab and select "Plain Text". 

If the plain-text test also fails or given the duration and impact, the most effective path is to contact Microsoft Support directly for further investigation and assistance. Here’s how to get the live chat support: 

1. Visit the Microsoft Support contact page: Contact - Microsoft Support 
2. In the search bar provided on the page, type "Contact support" and press Enter. 
3. Click on the "Get Help" button that appears. 
4. Scroll down the page until you see and click on the "Contact Support" option. 
5. When prompted to select your product or service, choose "Other Products". 
6. For the category, choose "Microsoft 365 and Office". 
7. Look for and click on "Chat with a support agent in your web browser". 
8. Confirm your personal email when prompted—this is the email Microsoft will use to communicate with you during and after the support session. 
9. Explain your situation clearly: Be concise but provide all the relevant details.   

Please try the plain-text test and then proceed to the chat support. Let me know the outcome of the test, or if you encounter any issues accessing the chat option. 

If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".   

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Hi Graziella, thanks for sharing the details. The error you’re seeing, “550 5.7.520 Message blocked because it contains content identified as spam,” means Outlook’s mail servers flagged something in your message or account as potential spam. This can happen if your email signature, links, or certain phrases trigger filters, or if your account reputation was affected by previous activity. First, check if your account is secure by running a full security scan and changing your password. Then, review your email content—avoid too many links, large attachments, or wording that looks promotional. Also, clear your Junk folder and make sure your account settings don’t include any suspicious rules or forwarding. Can you tell me if this happens with every recipient or only some, and whether you use any custom signature or links in your replies?

Thank you for this.

I will try later or tomorrow, as I had no opportunity to do all these trials due to night time here and other commitments today.

My emails are only work related and nothing suspicious, so I am really confused. I am an independent consultant and two months ago, all mails to my collaborators were bouncing back. Now some bounce back and some don't. I need to constantly check my spam to see if mails went through or not, and in some cases I try 3/4 times before it works with a new mail (which is not ideal as my colleagues do not have the thread), or I just use my gmail account.

My junk folder is always full of garbage and promotional materials, which I delete without opening. I also have several work-related mails to people I write to regularly that sometimes end up in my junk folder. So I took the habit to check daily. Also this is annoying as I have a paid account.

I will try to contact Microsoft.

Thank you again,