Look at the mail source that is delivered to the mail server and check that the mail is constructed properly. Microsoft Software has a long history of ignoring or misinterpreting internet standards, so you should make sure that you feed Outlook with a properly formatted mail to avoid any Microsoft guesswork.

I did not yet use Mail::Sender, but a quick scan of the documentation did not convince me to use it. Perhaps you could try MIME::Lite? It has its own set of minor problems, but I know that it reliably sends attachments that can be read by any MIME capable mail client.

Alexander