If you were looking to add unique names to each of your .Mac aliases, you probably stumbled upon an older hint here. However, directly editing plist files can be a little scary, and can result in corrupting Mail's preference file. The same result can be achieved much easier using this AppleScript -- no need to manually edit any files.

The script works for me on 10.4.6 -- you probably should make a backup of ~/Library -> Preferences -> com.apple.mail.plist before running the script nevertheless...

[robg adds: This worked for me, too, but only after I quit and relaunched Mail (it was running when I tried the script). The script tells you this isn't required, but I found it didn't work unless I did.]