powershell test/WhatIf question

I've written a script that will perform a get-mailbox, then add a new domain alias to each of the results.  However I want to test it.  I've seen google search results saying that this can be accomplished with a -WhatIf such as:

c:\>.\somescriptfile.ps1 -WhatIf

However that generate an error complaining about the whatif part.

Heres the code Im using:

$users = get-mailbox
foreach ($a in $users) {$a.emailaddresses.Add("smtp:$($a.alias)@new.domain.com")}
$users | %{set-mailbox $_.Identity -emailaddresses $_.emailaddresses}

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Not sure what I could put a -whatif statement in here... I tried on a new line at the bottom but it failed.