If you're sending sensitive docs to a colleague on the other coast, you don't want mail admins viewing them or nosy network gurus sniffing traffic.
A product called BetterZip, available for Mac OS X Snow Leopard, is a solution I often turn to. If you're an OS X Lion user, this feature comes built in.
BetterZip lets you make an archive of one or more documents and then protect the archive with encryption. You can then safely transmit the docs over the Internet via email.
These documents can also be shared with Windows users who use WinZip. (That's because BetterZip uses AES-256 in a format compatible with WinZip.)
Go to the macitbetter website and download BetterZip to a convenient location on your hard drive.
Once the download is complete, select the BetterZip icon and move it into your Applications folder.
Now head to your applications folder and start BetterZip.
After you tell BetterZip you would like to use the free 30 day trial, it will present you with a default window for starting an archive.
To create an archive, simply drag the files that you would like included into the window. Or click on the Add button if you prefer. Here I've added some company documents, which contain sensitive information that needs to be encrypted. Hit Save to create the archive.
After clicking Save, the program will prompt you for the name of the archive. Here is also where you select the encryption method. There are three settings: Not Encrypted, Weak Encrypted, and Strong (AES-256) WinZip Compatible.
Select Strong AES-256, which is the highest level of encryption offered.
Enter in a strong password twice to protect the archive.
You'll need to provide the recipient with the password. (You obviously don't want the password in the email message.) This is the password the recipient will need to open the archive.
Now just email the archive as you would any other attachment. Keep in mind that many corporate email servers limit attachments to 10MB. If you need to mail a larger archived file, you might need to split it up using a service like Dropbox.
If your recipient is a Mac OS X Lion user, he or she can open the archive with their copy of BetterZip.
If the recipient is on a Windows machine, he or she can use WinZip or 7zip to extract the files in the archive.
That's all there is to keeping your docs and emails safe and sound on Mac OS X. If you're running OS X Lion, check out its built in encryption feature.