I just created an extension for Betterbird (and Thunderbird 115 or newer) that creates a task in Remember The Milk from an email in two clicks, similar to the Remember The Milk add-on for Gmail. Features “RTM” button in the
Create tasks in “Remember The Milk” with Siri and the Apple Watch
I am a long year user of “Remember The Milk” (RTM) to manage all my tasks. Although they do still not have a native app on the iPhone, you are able to use Siri on the Apple Watch to automatically

