This guide explains how to create a tenant-wide mail flow rule in Microsoft 365 that marks messages containing [SPAM] in the subject line as junk so they are delivered to users' Junk Email folder.

This rule is configured in the Exchange Admin Center and applies to all users in your organization.

Step 1 — Open the Exchange Admin Center

1. Sign in to the Microsoft 365 Admin Center
   https://admin.microsoft.com

2. Open the Exchange Admin Center.

You can access it directly at:

https://admin.exchange.microsoft.com

This manages mail flow for Microsoft Exchange Online within Microsoft 365.

Step 2 — Navigate to Mail Flow Rules

1. In the left menu, click Mail flow

2. Click Rules

This page shows all transport rules that process email for your tenant.

Step 3 — Create a New Rule

1. Click Add a rule

2. Select Create a new rule

Fill in the following fields:

Name

Mark [SPAM] messages as Junk

Step 4 — Configure the Condition

Under Apply this rule if:

1. Select The subject or body includes

2. Choose Subject includes any of these words

3. Add:

[SPAM]

Click Add and then Save.

Step 5 — Configure the Action

Under Do the following:

1. Select Modify the message properties

2. Choose Set the spam confidence level (SCL)

Set the value to:

9

An SCL of 9 marks the message as high confidence spam, which causes Exchange Online to deliver it to the Junk Email folder.

Step 6 — Configure Rule Settings

Click Next

Recommended settings:

• Mode: Enforce

• Severity: Not specified

• Stop processing more rules: Optional but recommended

Step 7 — Save the Rule

Click Save.

The rule will now apply to all incoming messages. Any message with [SPAM] in the subject will have its spam confidence level set to 9, causing it to be delivered to the Junk Email folder.

How the Rule Works

When a message arrives:

1. Exchange scans the subject line.

2. If [SPAM] is present, the rule sets SCL = 9.

3. Exchange delivers the message to the user's Junk Email folder instead of the Inbox.