VERBOSE: [19:50:51.165 GMT] New-Mailbox : Unable to generate the e-mail address. Unable to load address module 'C:\Program Files\Microsoft\Exchange Server\V15\Mailbox\address\GWISE\AMD64\gwxpxgen.dll' for address type 'GWISE'. Additional message: 'The specified module could not be found'.
Resolution: There is a GWISE value still present in the Default Recipient Policy.
1. Open ADSI Edit.
2. Connect to the Configuration Naming Context of a Domain Controller or your domain.
3. Navigate to: Configuration --> CN=Services --> CN=Microsoft Exchange --> CN=<Exchange Name> --> CN=Recipient Policies
4. Right click on CN=Default Policy and select properties.
5. Locate the gatewayProxy attribute and remove the GWISE value.
Note: Other articles suggested looking at the disabledGatewayProxy as well and removing the GWISE value.