Hi John,
A few initial things you can check are as follows:
1. Ensure the two user accounts are not locked in AD
2. Ensure the two user accounts are not hidden from the GAL for Exchange
3. You can test creating a new receive connector by completing the below steps
Create an Exchange receive connector
1. Open the Exchange Management Console.
2. Expand Server Configuration.
3. Click Hub Transport.
4. Right-click Receive Connectors, and then click New Receive Connector.
5. Type a name in the Name box.
6. In the Select the intended use for this Receive connector list, click Internal, and then click Next.
7. In the Remote Network Settings section, click Add, and then type the IP address of the Microsoft Dynamics CRM server.
Note If you see the value 0.0.0.0-255.255.255.255, click Delete.
8. Click Next, click New, and then click Finish.
Configure the connector for the anonymous user
1. Right-click the Exchange receive connector that you created, and then click Properties.
2. Click the Permission Groups tab.
3. Make sure that the Specify who is allowed to connect to the Receive connector option is set to Anonymous users, click Apply, and then click OK.