Sidekick is one of the best mail utilities out there. Especially on OSX, combining the power of Apple Mail, and the possibility to track your email opens, it’s one of my favorite apps.
Unfortunately, the Hubspot team dropped support for Sidekick for Apple Mail, starting at OSX El Capitain. That said, the current app only does support OSX 10.11.0; Sidekick will cease to work if you update to OSX 10.11.1 or later (10.11.2 as of the time of this writing, on the stable channel.)
The video below explains exactly the steps to take to reactivate Sidekick for Apple Mail. Tested on OSX 10.11.0, 10.11.1, and 10.11.2 (respectively Apple Mail version 9.0, 9.1 and 9.2.)
PS: If you’re into the technical details, below the video is a brief explanation.
The SupportedPluginCompatibilityUUIDs key in the Sidekick Info.plist file, is sort of a manifest for the plugin, declaring itself compatible to Apple Mail client UUIDs (Universally Unique Identifiers.)
What we did was simply get the system’s current Apple Mail’s UUID (from /Applications/Mail/Info.plist), and add it to that list!