Microsoft 365 Error Codes

How to Fix Exchange Online Error 550 5.1.8 During Bulk Email

Khadija Khan Oct 23, 2026 869 views

Question details

The user needs to prevent or resolve Exchange Online error 550 5.1.8, which blocks the account when sending a large batch of emails.

How to Fix Exchange Online Error 550 5.1.8 During Bulk Email

Product: Exchange Online
Device & OS: not provided
Scenario: Sending a bulk email (e.g., 2,500 messages) to a company without exceeding the daily recipient limit, but triggering rate-limit restrictions.
Observed behavior: Exchange Online blocks the account and throws error 550 5.1.8 because the messages are being sent too quickly, exceeding the limit of approximately 30 messages per minute.

Before you start

Ensure you have administrator access to the Microsoft 365 Defender portal, as you may need to manually unblock your account if it has already been restricted.

Solution 1Recommended

Throttle or Schedule Bulk Email Sending

The most effective way to prevent error 550 5.1.8 is to control the rate at which emails are sent, keeping it below Exchange Online's threshold.

Exchange Online enforces a strict message-rate limit of approximately 30 messages per minute to prevent spam. Implementing Multi-Factor Authentication (MFA) improves your account security but does not bypass or increase these sending limits. There is no supported way to increase these limits in Microsoft 365.

Configure email throttling

Open your bulk email software or script settings and configure a sending delay. Ensure the sending rate is throttled to fewer than 30 messages per minute.

Schedule staggered send times

Divide your large recipient list (e.g., 2,500 contacts) into smaller batches. Schedule these batches to be sent automatically over several hours or days to avoid triggering the rate limit.

Alternative for High Volume: If your business frequently sends high volumes of bulk email, consider migrating those campaigns to a dedicated third-party bulk email service provider rather than using Exchange Online.

Solution 2

Unblock the Restricted Account in Microsoft 365 Defender

If you have already received error 550 5.1.8, your account is currently blocked from sending emails and must be unblocked by an administrator.

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Frequently Asked Questions

Does enabling Multi-Factor Authentication (MFA) increase my Exchange Online sending limits?

No, enabling MFA in Microsoft 365 enhances your account security against unauthorized access but does not change or relax Exchange Online's message-rate or recipient limits.

What is the exact message rate limit for Exchange Online?

Exchange Online generally restricts email sending to approximately 30 messages per minute. Exceeding this rate will trigger error 550 5.1.8 and restrict the account from sending further emails.

Can I request Microsoft to increase my Exchange Online sending limits?

No, there is no supported way to request an increase to these specific Exchange Online sending and rate limits. For high-volume bulk emailing, Microsoft recommends using a dedicated third-party bulk email service.

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