Send & Receive Issues

Fix Exchange Online Emails to Gmail Rejected as Unsolicited Mail

Maira Mehtab Oct 10, 2026 869 views

Question details

Users need to resolve an issue where emails sent from Microsoft Exchange Online to Gmail recipients are being blocked or rate-limited by Google.

How to Fix Exchange Online Emails to Gmail Rejected as Unsolicited Mail

Product: Microsoft Exchange Online
Device & OS: not provided
Scenario: Sending emails to one or multiple Gmail recipients from Exchange Online.
Observed behavior: Gmail rejects the messages with error 550 5.4.300 or temporarily rate-limits them, leaving messages pending or triggering undelivered notifications 24 hours later.

Before you start

Ensure you have administrator access to the Microsoft 365 Admin Center and the ability to modify your domain's DNS records.

Solution 1Recommended

Verify Domain Authentication and Contact Microsoft Support

Checking your SPF and DKIM records ensures proper domain authentication, while Microsoft Support can investigate and resolve potential IP reputation issues.

Because this issue affects Gmail recipients globally and triggers rate limits, it is highly likely that Microsoft sending IP addresses have been temporarily flagged by Google's spam filters.

Check DNS Records

Log in to your domain registrar's control panel and confirm that your SPF, DKIM, and reverse DNS records are correctly configured for Exchange Online.

Run a Message Trace

Open the Exchange Admin Center, navigate to Mail flow > Message trace, and run a trace for the affected emails to gather timestamps, recipient details, and the exact 550 5.4.300 error code.

Monitor Pending Messages

Check if the affected messages remain in a 'Pending' status. Note that undelivered notifications for this specific error typically arrive about 24 hours later.

Open a Support Request

Go to the Microsoft 365 admin center, navigate to Support > Help & support, and open a new ticket. Provide the support agent with the Gmail error, message trace results, timestamps, and recipient details to address the IP blocking.

Verify Domain Authentication and Contact Microsoft Support

Cross-Device Impact: This is a server-to-server issue. The error will persist whether you send the email from an iPhone, an iPad, Outlook desktop, or Office.com.

Solution 2

Limit Recipients per Email

A temporary workaround to help bypass aggressive rate limits when emailing Gmail users.

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Frequently Asked Questions

Why am I getting error 550 5.4.300 when emailing Gmail accounts?

This error signifies a delivery failure, usually occurring because Google's spam filters have temporarily rate-limited or blocked Microsoft Exchange sending IP addresses due to suspected unsolicited mail.

Does this email rejection happen if I use a mobile device?

Yes. Because the issue occurs at the Exchange server level between Microsoft and Google, the rejection happens regardless of whether you send from an iPad, iPhone, Outlook desktop app, or the Office.com web portal.

How long does it take to receive the undelivered notification?

When this specific rate-limiting occurs, messages typically remain in a 'Pending' state within the Exchange server. Undeliverable notifications (NDRs) are usually generated and sent to your inbox about 24 hours after the initial send attempt.

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