How To Validate Connectors In Exchange Online To Resolve Mail Flow Issues

In this post I am going to show you how you can validate your connectors in Exchange Online to troubleshoot mail flow issues. This is an important troubleshooting step if we are routing mail to and from Exchange Online with partner organisations or if we are using 3rd party services. This post make up part of my full MS-220 Troubleshooting Exchange Online exam study guide.

Why do we need connectors in Exchange Online

Exchange (both Exchange Online and Exchange sever use connectors to control inbound and outbound mail flow (email that the server receives from an external source and email that the server sends). 

Receive connectors – Control incoming mail. They listen and wait for incoming emails that match the configuration of the connector. For example emails that come from a specific public IP. A common scenario would be when using a 3rd party mail filtering/security solution.

Send connectors – Control the outbound mail from a server based on a desired configuration. For example, mail from a specific internal subnet. They allow you to route email to a specific location, for example, a 3rd party mail filter, or email encryption solution.

Delivery Agent Connectors – Control outbound mail flow to non-SMTP based systems. An example of this would be a fax gateway server.

Foreign connectors – These are similar to delivery agent connectors and are used to send mail to non-smtp based mail systems, however you must configure a drop directory as it used file-transport mechanisms instead of SMTP.

How to validate connectors in Exchange Online

1. Login to with your Global admin or Exchange admin user

2. From the left hand menu, select Mail flow > Connectors

3. Select your existing connector

4. A new window will appear from the right hand side, under Validation, select Validate this connector

5. Enter an email address for a mailbox located on that mail server and click Validate

6. You will either see validation successful or validation failed

What errors will users get if Connectors are not working?

There are various errors you will see if connector validation failed. Here are a few common errors and their meanings. 

The domain of the recipient is not configured as part of connector.

    • This means the domain you are testing has not been added to the accepted domains list.

550 5.4.310 DNS domain does not exist

    • In this instance the FQDN of the mail server does not exist or has not yet propagated if only recently created.

5.7.1. Unable To Relay: Blocked by Customer Allow list

    • The external email server IP address is on an real-time block list that is used by Exchange Online Protection
To see further errors and troubleshooting steps, check out the Microsoft Docs.


Thank you for taking the time to read my post on how to validate connectors in Exchange Online. This has hopefully been particularly useful and helped you with your study towards the MS-220 exam. Be sure to check out some of my similar posts:

Daniel Bradley

My name is Daniel Bradley and I work with Microsoft 365 and Azure as an Engineer and Consultant. I enjoy writing technical content for you and engaging with the community. All opinions are my own.

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