Apple Mail Attachments

If you are having problems using email to mail with your Apple Mail client, make sure you enable sending of Windows-friendly attachments. 

Instructions from Apple support page:

To do so for all messages, choose Edit > Attachments, then select Always Send Windows-Friendly Attachments. For a specific message, click the Attach button in the toolbar of the new message window, then select Send Windows-Friendly Attachments (if you don’t see the checkbox, click Options in the bottom corner).