Send & Receive Issues

How to Fix Office 365 SMTP Relay Blocked for Suspected Brute-Force Activity

Muhammad Talha Oct 22, 2026 868 views

Question details

Users are experiencing temporary denial of Office 365 SMTP relay requests due to suspected brute-force activity when sending large volumes of messages.

How to Fix Office 365 SMTP Relay Blocked for Suspected Brute-Force Activity

Product: Office 365, Business Central
Device & OS: Windows Server
Scenario: Sending multiple emails or automated messages through Business Central and a Windows server via the Office 365 SMTP relay.
Observed behavior: SMTP relay requests are temporarily denied or blocked by Office 365 security filters, even though no actual brute-force attack is occurring.

Before you start

Ensure you have administrator access to your Office 365 tenant and Exchange admin center to review mail flow logs, connectors, and security alerts.

Solution 1Recommended

Gather Error Details and Contact Microsoft Support

Since Microsoft's automated security systems block the relay, collecting exact error logs and contacting support is the most effective way to investigate and lift the restriction.

Office 365 automatically throttles or blocks email traffic that mimics spam or brute-force patterns. To resolve this, you must gather specific metrics about your mail flow before reaching out to Microsoft.

Collect exact error messages

Note down the specific NDR (Non-Delivery Report) codes or SMTP error messages returned when your emails fail to send from the server.

Identify affected parameters

Record the affected recipient domains, timing of the blocks, message volume per hour, and whether the delivery failures happen randomly or consistently.

Open a support ticket

Log into the Microsoft 365 admin center, navigate to the 'Support' section, and create a new service request. Provide the gathered information to the support agent for investigation.

Gather Error Details and Contact Microsoft Support

Data Privacy: Always share sensitive server logs and routing details through secure, private messages with official support rather than posting them on public forums.

Solution 2

Review Connector and Sending Limits

Ensure your email volume does not exceed Exchange Online's built-in receiving and sending limits to prevent automated blocks.

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Frequently Asked Questions

Why does Office 365 think my SMTP relay is a brute-force attack?

Microsoft 365 employs strict automated anti-spam and anti-malware filters. If a large volume of automated emails is sent in a short burst from a single IP or connector, the system may flag it as suspicious activity and temporarily block the relay to protect the network.

How can I prevent my Business Central emails from being blocked?

Ensure your Exchange connector is configured correctly, authenticate using a dedicated mailbox, use TLS encryption, and throttle your outbound message volume to stay well below Microsoft's sending limits of 30 messages per minute.

Can I use a third-party SMTP provider instead of Office 365?

Yes, for high-volume automated transactional emails, it is often recommended to use dedicated third-party SMTP delivery services rather than routing bulk automated mail through an Office 365 mailbox.

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