Outlook Error Codes

Fix Outlook Error 550 5.7.520: Message Blocked as Spam

Chanuka Geekiyanage Oct 20, 2026 870 views

Question details

The user is unable to send an email from a Hotmail/Outlook account because it bounces back with an error indicating the message contains spam-like content.

How to Fix Outlook Error 550 5.7.520 (Message Blocked as Spam)

Product: Microsoft Outlook
Device & OS: not provided
Scenario: Sending an email message that includes specific formatting, links, or attachments to an external recipient.
Observed behavior: The email is rejected by the server and returns a Non-Delivery Report (NDR) with Outlook error 550 5.7.520, stating 'Message blocked because it contains content identified as spam.'

Before you start

Before modifying your email settings, try sending a completely blank test email with a simple subject line (like 'Test message') to the same recipient to verify if the block is triggered by your account standing or just the contents of the previous email.

Solution 1Recommended

Adjust Email Formatting and Remove Attachments

Modify the content of your outgoing email to bypass the automated spam filters that are incorrectly flagging your message.

Error 550 5.7.520 is frequently triggered by Microsoft's outbound spam filters when they detect suspicious formatting, spam-associated keywords, or problematic attachments like the proprietary winmail.dat format. Adjusting your email composition can quickly resolve this.

Change message format

Switch your email composer from Rich Text Format (RTF) to either HTML or Plain Text. In Outlook, you can change this in the Format Text tab on the ribbon.

Remove problematic attachments

Delete any winmail.dat files or unnecessarily large attachments from the email. If you need to share large files, consider sending a secure cloud storage link instead.

Clean up the email body

Review the email content and remove suspicious URL links, excessively promotional wording, or broken HTML code that might trigger a spam filter.

Resend the email

Once the formatting is simplified and attachments are removed, try sending the message again to see if it successfully passes the outgoing server filters.

Adjust Email Formatting and Remove Attachments

Outlook on the Web: If you are using the Outlook desktop app, try logging into Outlook.com via your web browser and sending the exact same message. This can bypass local client synchronization issues that sometimes cause false spam flags.

Solution 2

Verify Account Security and Request Support

Confirm that your account has not been compromised and reach out to the email administrator to lift any server-side restrictions.

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Frequently Asked Questions

Does Error 550 5.7.520 mean my entire email account is permanently blocked?

Not necessarily. In most cases, this error indicates that a specific email was flagged by outbound spam filters due to its content, formatting, or attachments. However, if your account was compromised and used to send bulk spam, Microsoft may temporarily restrict your sending capabilities until you secure the account.

Why does my email send successfully from Gmail but fail from Outlook?

Different email providers use varying spam filtering algorithms and reputation checks. Microsoft's outbound spam filters (which apply to Hotmail and Outlook) might flag certain content, IP addresses, or proprietary attachments like winmail.dat that Gmail's servers process without issue.

How do I stop Outlook from automatically sending winmail.dat attachments?

To prevent Outlook from sending winmail.dat files, go to File > Options > Mail. Under the 'Compose messages' section, change 'Compose messages in this format' to HTML or Plain Text. Scroll down to 'Message format' and ensure 'When sending messages in Rich Text format to Internet recipients' is set to 'Convert to HTML format'.

Microsoft Office Error Codes