If you recently updated to Snow Leopard 10.6.5 you can see an alert at starttime of Mail. In this alert Mail says that he disabling some plugins. At my MBP the candidates were Letterbox, Herald and GPG. Bit there is a quick workaround for this:
Navigate with finder to /Users/USER_NAME/Library/Mail. There are many directories. Here you search for the disabled mailbundle. Some of the directories are namend Bundle (deactivated*). In this kind of directory Mail moved the plugins. After you found the mailbundle open it with the command Show Contents and navigate to the Info.plist. Open it with a double click and add to entries to the section SupportedPluginCompatabilityUUID. The both entries must have this values:
Close the PList-Editor and restart Mail. All plugins should be enabled.