The code below will show you how to remove a specific domain from all users mailboxes.
The code was tested on Exchange 2010 and Exchange 2013.
#REMOVE_DOMAIN.PS1
if ( (Get-PSSnapin -Name Microsoft.Exchange.Management.PowerShell.E2010 -ErrorAction SilentlyContinue) -eq $null )
{
add-pssnapin Microsoft.Exchange.Management.PowerShell.E2010
}
foreach($i in Get-Mailbox -ResultSize Unlimited) {
$i.EmailAddresses |
?{$_.AddressString -like ‘*@ntweekly.com’} | %{
Set-Mailbox $i -EmailAddresses @{remove=$_}
}
}