'Hypocritical' Error when Running New-Mailbox

This is a cross post from /r/Powershell, then I realised I’ve never used the main globalcommunity! Hi everyone, look forward to working with you all.

Down to business…

I’m running:

 New-Mailbox -Alias alias.alias -Name "WhiteSpace Support" -FirstName ***** -LastName ***** -DisplayName "Display name" -UserPrincipalName "blah@blah.com" -Password (ConvertTo-SecureString -String 'Pa$$word1' -AsPlainText -Force) 

Which gives out the error:

  A parameter cannot be found that matches parameter name 'UserPrincipalName'. 

When I remove the -UserPrincipalName parameter, I get asked to define the UserPrincipalName manually.
What gives?

What does this command gives you back:

Get-Command New-Mailbox -Syntax

Ah, so the Syntax path I followed doesn’t include that parameter, so need to define it afterwards.