First, the recipient needs Final Draft installed on his/her computer. If it's not, they can download the free FD10 Trial, HERE, which will act as a reader. The other option is to save the file as a PDF; the command is File > Save as PDF.
If you have FD 9 / 8 and your colleague has FD 7 / 6 / 5, you will need to save a copy of the script in the .fdr file format. Go to File > Export (FD9) or File > Save As (FD8) and in the Save as Type dropdown (Windows) or the Format dropdown (Mac) choose Final Draft 5-7 Document.
Final Draft 10 does not save scripts as FDR files.
If you're able to open the script on your computer but your colleague can't open it on theirs, the file may be corrupted. It may have been corrupted before you emailed it and the emailing made it worse, or it may have become corrupted when you emailed it. Click HERE for information on how to create a healthy new copy of your file. Email this new one and see if your colleague still can't open it.
Emailing a file of any kind can corrupt it. The-mail program you use can corrupt it and so can the internet provider on either end.
To combat this and to keep the size of the file down, certain ISPs (especially AOL) and certain email programs compress attached files automatically. There are several compression formats used: stuffed, zipped, binhexed, mimed, and others. If the person on the receiving end doesn't have the proper software to decompress the file, they won't be able to open the file or it willl appear as coded text.
To counteract this, it's a good idea to compress the file before you attach it to the email using a method built into the Mac and Windows operating systems. For information on how to easily compress your script files, click HERE.