Exchange Connection Problems

How to Fix Reserved IP Address Errors When Creating an Exchange Connector

Rana Garcia Sep 30, 2026 870 views

Question details

The user is encountering an error stating that an IP range contains reserved IP addresses when attempting to create or configure an Exchange Online SMTP relay connector.

Fix Reserved IP Address Errors When Creating an Exchange Connector

Product: Microsoft Exchange Online
Device & OS: not provided
Scenario: Setting up an inbound SMTP relay connector in Microsoft Exchange Online using an IP address.
Observed behavior: The system rejects the configuration, returning an error that the specified IP range contains reserved or private IP addresses, which are invalid for this connector type.

Before you start

Ensure you have Microsoft 365 Exchange Administrator privileges and know your organization's exact static public IP addresses before modifying connector routing settings.

Solution 1Recommended

Update the Connector with a Valid Public IP Address

Replace any local, private, or reserved IP addresses in your connector settings with a fully routable, static public IP address.

Exchange Online requires a valid public source IP for authentication on SMTP relay connectors. Private IP ranges (such as 10.x.x.x, 172.16.x.x, or 192.168.x.x) are strictly blocked because they cannot be verified over the public internet.

Access Exchange Admin Center

Log in to the Microsoft 365 portal and open the Exchange admin center. Navigate to the 'Mail flow' menu and select 'Connectors'.

Edit the Inbound Connector

Locate the specific SMTP relay connector causing the error, click on it, and select 'Edit' under the 'Authenticating sent email' or IP address configuration section.

Remove Reserved IPs

Review the list of entered IP addresses. Select and delete any private or reserved IP ranges from the configuration.

Add Your Public IP

Enter your organization's static, non-reserved public IP address or public IP range, then click 'Save' to apply the new settings.

Update the Connector with a Valid Public IP Address

Verification Tip: If you are unsure of your outward-facing IP, use an online IP checker tool from a server within your network to verify your true public IP.

Solution 2

Verify the Sending Source IP in Third-Party Gateway Logs

Use this method if your emails route through a third-party security gateway (like Proofpoint) before reaching Exchange.

Solution 3

Escalate to Microsoft 365 Support

If you have confirmed your IP is public but the system still incorrectly flags it as reserved, Microsoft support intervention is required.

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Frequently Asked Questions

What is considered a reserved IP address in Exchange Online?

Reserved or private IP addresses are ranges designated for local networks and cannot be routed across the public internet. Common examples include 10.0.0.0 to 10.255.255.255, 172.16.0.0 to 172.31.255.255, and 192.168.0.0 to 192.168.255.255. Exchange Online rejects these for public connector configurations.

Why does an SMTP relay connector require a public IP address?

Exchange Online relies on public IP addresses to verify and authenticate the source network of incoming emails. This security measure prevents spam, spoofing, and unauthorized open relaying from internal networks.

How do I find the correct public IP for my Exchange connector?

You can identify your public IP by checking the outbound logs of your firewall, router, or email security gateway (like Proofpoint). Alternatively, you can use an online 'What is my IP' service from the server handling your outbound SMTP traffic.

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