Hi, Roger Roger
With the command you provided, the syntax is correct through my tests, and I have successfully created a mail contact. The result is shown below.
When you create an alias for external users in your domain, the resulting email address typically follows the primary domain format that you set up for your organization in Microsoft 365.
If your primary domain is contoso.com, then the alias you create will often generate an email address like someuser.ext@contoso.com.
If my answer is helpful to you, please mark it as the answer so that other users can refer to it. Thank you for your support and understanding.