Send & Receive Issues

Fix Microsoft 365 Email Rejected as Suspected Spam (Error 550 5.7.350)

Camila Milosovich Oct 13, 2026 868 views

Question details

External mail servers are rejecting emails sent from Microsoft 365 due to suspected spam, returning error 550 5.7.350.

Fix Microsoft 365 Email Rejected as Suspected Spam (Error 550 5.7.350)

Product: Microsoft 365
Device & OS: not provided
Scenario: Sending emails to external recipients from Microsoft 365.
Observed behavior: Emails bounce back with SMTP error 550 5.7.350 indicating they were rejected as suspected spam by the recipient's mail server or filtering service.

Before you start

Ensure you have a copy of the Non-Delivery Report (NDR) or bounce message, as it contains critical diagnostic information needed to resolve the issue.

Solution 1Recommended

Isolate the issue and test a plain text email

Determine if the issue is caused by your email content (attachments, signatures, formatting) or if it affects specific recipients.

Often, complex signatures, embedded links, or certain file attachments can trigger sensitive spam filters on the recipient's mail server.

Send a plain text test

Create a new email without any attachments, HTML formatting, links, or email signatures. Send this plain text message to the affected recipient.

Determine the scope of the problem

Check if the test email successfully delivers. Also, try sending the same plain text email to a completely different external domain to see if the issue is widespread.

Request domain whitelisting

If the plain text email is delivered successfully to other domains but still fails for one specific recipient, contact that organization's email administrator through an alternative method and ask them to allowlist your sender address or domain.

Isolate the issue and test a plain text email

Solution 2

Create a new message draft

Use this workaround if specific hidden formatting or corruption in the original email draft is causing the spam filter to trigger.

Solution 3

Escalate to Microsoft Support

If all external recipients are rejecting your emails with error 550 5.7.350, your domain or tenant IP may be broadly blocked and requires Microsoft intervention.

Free Microsoft Office alternative

Looking for a Lightweight Alternative to Microsoft Office?

While resolving Microsoft 365 email routing issues, consider WPS Office for your daily document creation needs. It is a free, fast, and highly compatible alternative to Microsoft Office, ensuring your attached documents are perfectly formatted and easily shared.

Fully compatible with Microsoft Word, Excel, and PowerPoint formatsLightweight installation with fast startup timesCreate clean PDFs and documents that are less likely to trigger email spam filtersFamiliar, easy-to-use tabbed interface for seamless migration

microsoft office alternative - wps office

Frequently Asked Questions

What does SMTP error 550 5.7.350 mean?

This error code indicates that the receiving email server or its third-party spam filtering service has rejected your message because it suspects the content, attachments, or sender is spam.

Can my email signature cause a spam rejection?

Yes, email signatures containing multiple external links, promotional images, or domains that have been flagged by security services can easily trigger sensitive spam filters on the recipient's end.

Why do emails with certain attachments get blocked as spam?

Spam filters often block messages with unusually large attachments, executable file types, or documents containing embedded macros as a strict security precaution against malware distribution.

Outlook & Email Problems