Fix Microsoft 365 Emails Rejected as Spam for One Recipient
Question details
The user is encountering NDR error 550 5.7.350 because their emails are being rejected as spam by one specific recipient, despite the sender domain and address being on standard allowlists.
- Product
- Microsoft 365
- Device & OS
- not provided
- Scenario
- Sending outbound emails to a specific customer or client.
- Observed behavior
- Messages sent to one particular recipient fail to deliver, returning a Non-Delivery Report (NDR) with error code 550 5.7.350 indicating the message was rejected as spam by the receiving server.
Verify that your emails are successfully delivering to other recipients and external domains to confirm that this is an isolated block rather than a global issue with your Microsoft 365 tenant.
Contact the Recipient's Email Administrator
Because the issue affects only one recipient, the rejection is caused by their specific inbound spam-filtering system. You must request that their IT department allowlists your sending details.
Error 550 5.7.350 specifically indicates that the recipient's local security policy or third-party spam filter (such as Proofpoint or Mimecast) actively blocked your message. Modifying your own Microsoft 365 settings will not override the recipient's inbound security rules.
Contact the intended recipient using a phone call, messaging app, or a completely different email address (such as a personal account).
Inform the recipient that your emails are bouncing back with NDR error 550 5.7.350 due to their organization's spam filters.
Ask the recipient to forward this issue to their email administrator or IT support team.
Instruct their administrator to add your sender IP address, email address, or entire domain to their spam-filtering and safe-sender policies.
Optimize Your Email Content to Avoid Spam Triggers
If you cannot immediately contact the recipient's IT department, you can try altering the content of your email to see if it bypasses their automated spam filters.
Try WPS Office for Seamless and Secure Document Sharing
While resolving Microsoft 365 email delivery issues, consider switching to WPS Office. It is a lightweight, fully compatible alternative to Microsoft Office. Instead of sending bulky email attachments that trigger strict spam filters, WPS Office allows you to create high-quality documents and securely share them via cloud links, ensuring your clients always receive your files without email bounce-backs.
- 1. Download and Install WPS Office: Get the free WPS Office suite from the official website and install it on your PC or Mac.
- 2. Create or Import Your Document: Open your existing Microsoft Office files (.docx, .xlsx, .pptx) in WPS Office without losing any formatting.
- 3. Share via Link: Click the 'Share' button to generate a secure link to your document, which you can safely send in the body of a plain-text email to bypass spam filters.
Frequently Asked Questions
What does NDR error 550 5.7.350 mean?
This specific error code indicates that the destination email server actively rejected your message because it flagged the content, sender address, or sending IP as spam according to its local inbound security policies.
Why is only one specific recipient blocking my emails?
Different organizations utilize different enterprise spam filtering services (such as Proofpoint, Mimecast, or custom Exchange rules). If your email inadvertently triggers a rule unique to their specific system, it will be rejected even if all your other clients receive your emails normally.
Can I bypass the recipient's spam filter without contacting their IT department?
Generally, no. While removing attachments, links, or complex HTML formatting might help you bypass lower-level automated content filters, an explicit domain reputation block or strict policy on their end can only be resolved by their email administrator modifying the safe-sender list.
