Starting with Exchange 2013, Microsoft changed the Exchange FrontEnd Transport service so that it no longer rejects invalid recipients during the RCPT phase of the SMTP conversation, which breaks our recipient verification.

To work around this, please follow these instructions:

1. Ensure the Exchange Anti-Spam Agents are installed. You can check via Exchange Management shell with the following EMS command:Get-TransportAgent

   Make sure that Recipient Filter Agent is listed and enabled. If it is not, install it with the following command:

   & $env:ExchangeInstallPath\Scripts\Install-AntiSpamAgents.ps1

2. Ensure that the Recipient Filter Agent is enabled. Use the following command:Enable-TransportAgent "Recipient Filter Agent"
3. Enable AddressBook. All of your domains need to be using Address Book to check for recipients. To check, run the following command:Get-AcceptedDomain | Format-List Name,AddressBookEnabled

   If Address Book is disabled for any domains for which the Exchange server is authoritative, fix that with the following command, once for each such domain:

   Set-AcceptedDomain name_of_domain -AddressBookEnabled $true

4. At this point, restart the Microsoft Exchange Transport service.
5. Ensure that Recipient Validation is enabled. Run the following command:Set-RecipientFilterConfig -RecipientValidationEnabled $true

   and again, restart the transport service.

6. Allow access to the Default receive connector. In the Exchange Administrative Center, go to Mail Flow : Receive Connectors. Edit the default connector, go to the Security tab and ensure that anonymous users are allowed. This lets us use the connector for recipient verification.
7. Make sure that port 2525 on your Exchange server is open to all the IPs we have provided you with. If the Exchange server is behind a firewall, you may also need to make a port-forwarding rule.
8. Test that Recipient Filtering works. Open a telnet session to port 2525 of your Exchange server. Type the bold text; the server responses should be similar to those shown below. Replace example.com with an actual domain name hosted on the Exchange server.
220 myexchange.example.com Microsoft ESMTP MAIL Service ready at...
HELO example.com
250 myexchange.example.com Hello ...
MAIL From:<devnull@roaringpenguin.com>
250 2.1.0 Sender OK
RCPT To:<nonexistent_user@example.com>
550 5.1.1 User unknown

9. Contact us to let us know when this is done, so we can complete your account setup.