The issue is not that Final Draft is incompatible with Hotmail, Yahoo!, etc. The issue is with Hotmail (or any free web-based e-mail like Yahoo, etc.) and formatted files. While web-based e-mail is great for standard e-mailing, there's a reason it's free. Although most free e-mail services and servers are designed to recognize standard file formats like jpegs, .doc files, etc. they're about the least optimal way to e-mail complicated formatted files. These files can get tweaked, especially file formats that are not universally recognized like Final Draft's.
The best way to e-mail formatted files using free web-based servers is to compress the file before you attach it to the email. This puts a protective shell around the file which keeps it from being altered.
On Windows 8 / 7 / Vista / XP, right-click on the file and choose Send To > Compressed (Zipped) Folder. You will see a compressed copy of the file in the same folder as the original; attach the copy to the email.
On Mac OS X 10.8 / 7 / 6: Control-click on the file and choose Compress "Filename."
On Mac OS X 10.4 / 3: Control-click on the file and choose Create Archive of "Filename."
You will see a compressed copy of the file in the same folder as the original; attach the copy to the email.
When the file is opened on Windows 8 / 7 / Vista / XP or OS X it will automatically be decompressed and openable. Users of older operating systems can use freeware or shareware compression programs such as Stuffit or WinZip.