Restricting Email Communication to One Domain in Microsoft 365 (Exchange Online)

Objective

Allow [e-mail] to send and receive emails only to/from @icicibank.com. All other email communications (inbound or outbound) should be blocked.

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Steps to Configure in Exchange Admin Center (EAC)

1. Login

• Go to https://admin.exchange.microsoft.com

• Sign in with your admin credentials.

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2. Go to Mail Flow

• Navigate to Mail flow → Rules.

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3. Create a New Rule for Inbound Mail (Receiving)

• Click “Add a rule” → “Create a new rule”.

Name: Allow only icicibank.com to send to [e-mail]

Apply this rule if...

• The recipient is → [e-mail]

Do the following...

• Reject the message with the explanation: “This mailbox accepts mail only from @icicibank.com domain.”

Except if...

• The sender domain is → icicibank.com

Mode:

• Enforce (Active)

Save the rule.

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4. Create a New Rule for Outbound Mail (Sending)

• Click Add a rule again.

Name: Allow [e-mail] to send only to icicibank.com

Apply this rule if...

• The sender is → [e-mail]

Do the following...

• Reject the message with the explanation: “This mailbox is restricted to send only to @icicibank.com domain.”

Except if...

• The recipient domain is → icicibank.com

Mode:

• Enforce

Save the rule.

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5. Test

• Try sending an email from [e-mail] to:

  • ✅ [e-mail] → Should go through.

  • ❌ [e-mail] → Should be rejected.

• Test inbound by sending:

  • ✅ From [e-mail] → Should arrive.

  • ❌ From [e-mail] → Should bounce with your custom error.

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6. Optional: Audit & Documentation

• Record the rule names and descriptions.

• Note the date/time of implementation.

• Optionally, export the rules from Exchange Admin Center for backup.

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Summary

Direction	Allowed Domain	Blocked	Mailbox Affected
Inbound	@icicibank.com	All others	[e-mail]
Outbound	@icicibank.com	All others	[e-mail]