Howto check if a powershell command succeeded or not?

Read Set-CASMailbox reference:

• OwaMailboxPolicy parameter:

The OwaMailboxPolicy parameter specifies the Outlook on the web mailbox policy for the mailbox. You can use any value that uniquely identifies the Outlook on the web mailbox policy. For example:

• Name
• Distinguished name (DN)
• GUID

The name of the default Outlook on the web mailbox policy is Default.

• Cmdlet Input and Output Types. If the Output Type field is blank, the cmdlet doesn’t return data (it's the Set-CASMailbox case).

Read about_CommonParameters (the parameters that can be used with any cmdlet), apply either ErrorVariable or ErrorAction:

ErrorVariable:

Set-CASMailbox -Identity:blocks.5 -OWAMailboxPolicy "DoNotExists" -ErrorVariable test
if ($test.Count -neq 0)      ### $test.GetType() is always ArrayList
{
    Write-Host "The Set-CASMailbox command failed: $test"
}
else
{
    Write-Host "The Set-CASMailbox command completed correctly"
}

ErrorAction and Try,Catch,Finally (read about_Try_Catch_Finally how to use the Try, Catch, and Finally blocks to handle terminating errors):

try {
    Set-CASMailbox -Identity:blocks.5 -OWAMailboxPolicy "DoNotExists"  -ErrorAction Stop
                ### set action preference to force terminating error:  ↑↑↑↑↑↑↑↑↑↑↑↑ ↑↑↑↑
    Write-Host "The Set-CASMailbox command completed correctly"
}  
catch {
    Write-Host "The Set-CASMailbox command failed: $($error[0])"  -ForegroundColor Red
}

In any case, read Write-Host Considered Harmful.