Then you will need a program to edit info.plist files. If you have developer tools installed you will use Property List Editor if not you will need a program that will edit an .xml file like my favorite Texwrangler.
PRIOR TO THE FOLLOWING PLEASE CLOSE MAIL.APP
Next you have to find and edit the info.plist for letterbox. Yours may be a little bit different but by default the file we need is in You User Folder -> Library -> Mail-> Bundles. If you have opened mail and got the disabled warning message your folder will have “(Disabled)” added to it. For now Leave this alone. Open the folder and you will see a list of the mail plugins you have .
Right click on the Letterbox.mailbundle and select “Show Package Contents” a new Finder window will open and you will see a contents folder, open this and you will see the info.plist file we need to edit. Open it in Textwrangler.app and scrool to the bottom of the file. There you will see the area we need to edit.
Look for (or use the find command) the line <key>SupportedPluginCompatibilityUUIDs</key>.
Once you find this you need to add the 2 strings from above to it. or just copy the next 2 lines and paste it in the file just under the other <string> tags as you see in my screen grab from above, make sure it is before the tag.
Now we need to deal with the “(Disabled)” part of the folder. This is easy just go to the finder and open back to the Mail folder from above and edit the folder name and delete the “(Disabled)” part. If you have other mail plugins you can edit the info.plist file the same way,If you choose not to edit them they will disable again on the next launch of Mail.app. If you have this all done then when you open mail you will be back in business with mail.app and the letterbox plugin.
**** UPDATE***** (11/12/2010 @ 11:07 am est)
I have had 3 people contact me with issues after they edit the info.plist file. I have asked them all to send me a copy of the info.plist file on their machine and all have had either old versions or a very different items in the file. I have asked all of them to re-download the plugin from the original source. (Letterbox Plugin @ Harnly.net)Then prior to opening Mail.app do the edit as above. So Far that has worked for anyone who has had an issue.