Hi Larry,
From the information above, I found that you are using Command Prompt not Windows PowerShell.
To find the Windows PowerShell, you can refer to the screenshot.
If you couldn’t find it, please refer to the article : Install and Configure Windows PowerShell.
After opening the Windows PowerShell, run the 5 steps in the article to connect it to your Office365 service.
At last, run the command “Get-mailbox | where {$_.emailaddresses -like "smtp:user@doamin.com"} | ft name, RecipientType, EmailAddresses”.
Thanks,
Ida Qiu