Exchange Connection Problems

How to Fix Exchange Online Connector Error 450 4.4.316

Adam Davis Oct 12, 2026 870 views

Question details

The user needs to resolve an email delivery delay caused by an Exchange Online connector failing to reach a destination server.

How to Fix Exchange Online Connector Error 450 4.4.316

Product: Exchange Online
Device & OS: not provided
Scenario: Routing emails from Exchange Online to an older Domino server using a dedicated connector.
Observed behavior: The connector returns error 450 4.4.316 (connection refused, socket error 10061), causing message delays even though firewall communication appears to be permitted.

Before you start

Ensure you have administrative access to the receiving Domino server's network settings and the Microsoft 365 Admin Center before modifying any routing or firewall rules.

Solution 1Recommended

Verify Domino Server and Network Configuration

Check the destination server's network environment, as a connection-refused error typically indicates a receiving-server or network blockage.

Error 450 4.4.316 with socket error 10061 means the target machine actively refused the connection. This usually happens when the SMTP service is down or network layers (like NAT or firewalls) are blocking the traffic.

Check the SMTP listening service

Log into your older Domino server and verify that the SMTP service is currently running and actively listening for incoming connections.

Test port 25 accessibility

Use a network diagnostic tool (like Telnet) from an external source to confirm that port 25 on the Domino server's public IP address is open and responding.

Review firewall and NAT rules

Examine your perimeter firewall and NAT routing configurations to ensure traffic from Exchange Online IP addresses is properly routed to the internal Domino server.

Verify Domino Server and Network Configuration

Temporary Error State: The '450' code is a transient error, meaning Exchange Online will keep the delayed messages in a queue and retry delivery automatically once the connection issue is resolved.

Solution 2

Escalate to Microsoft 365 Support

If local network diagnostics show no issues, contact Microsoft Support to inspect tenant-level routing logs.

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Frequently Asked Questions

What does socket error 10061 mean in Exchange Online?

Socket error 10061 indicates that the target server actively refused the connection. In the context of an Exchange connector, it usually means the required SMTP service on the receiving Domino server is either not running, or a firewall/NAT rule is actively blocking port 25.

Why are my emails delayed instead of failing immediately?

The error code begins with '450', which represents a transient (temporary) failure in the SMTP protocol. Exchange Online recognizes this as a temporary network or server glitch and places the emails in a retry queue rather than immediately bouncing them back to the sender.

Can I fix error 450 4.4.316 from the Exchange Admin Center?

Usually, no. Because a connection-refused error points to a destination network problem, the issue must typically be fixed on the receiving end by adjusting the older Domino server's SMTP settings or the perimeter firewall.

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