Below powershell helps to pull out all the  secondary email address for the given set of users

Get-Content C:\users.txt|Get-Mailbox | foreach {
for ($i=$_.EmailAddresses.Count;$i -ge 0; $i–)
{
$_.EmailAddresses[$i].ProxyAddressString

if ($_.EmailAddresses[$i].ProxyAddressString  -like “smtp:*” )
{
$_.EmailAddresses.RemoveAt($i)
}

}
$_|set-mailbox
}