Remove SMTP address for a specific domain from all mailboxes.
Here is a handy script to remove obsolete proxyaddresses from all mailboxes.
foreach($i in Get-Mailbox -ResultSize Unlimited) {
$i.EmailAddresses |
?{$_.AddressString -like '*@domain.com'} | %{
Set-Mailbox $i -EmailAddresses @{remove=$_}
}
}
Just replace the domain.com with your own.