If you use [GPGMail](https://gpgtools.org) (look it up if you don't know about email privacy ;)), and migrated to OSX Mavericks preview, then you know it does not work out of the box.
Here's how to trick the operating system to make it work:
- Quit Mail; - Download and [install this fix](http://support.gpgtools.org/discussions/everything/10837/r?go=aHR0cHM6Ly90LmNvL0VESlBRUVFUVGw=); - Run this sequence of commands:
$ sudo defaults write /Library/Mail/Bundles/GPGMail.mailbundle/Contents/Info.plist SupportedPluginCompatibilityUUIDs -array-add "$(grep -A 1 "PluginCompatibilityUUID" /Applications/Mail.app/Contents/Info.plist | grep string | cut -f 2 -d ">" | cut -f 1 -d "<")" $ sudo chown -R root:staff /Library/Mail/Bundles/GPGMail.mailbundle $ sudo chmod -R ug+r /Library/Mail/Bundles/GPGMail.mailbundle $ sudo chmod 644 /Library/Mail/Bundles/GPGMail.mailbundle/Contents/Info.plist
Keep in mind that on some shells the first command will not work (i.e. [fish](http://fishshell.com)). If that's the case, first switch to another one (bash,sh,csh,zsh) then run the commands.
Restart Mail and it should work.