Thunderbird is a free desktop email client for Windows, Mac, and Linux. Despite free, Thunderbird is qualified enough for business use. The features offered by Thunderbird are not much different to paid email client apps like Outlook and Mailbird. One of the features offered by Thunderbird is the ability to turn an email into a task.
Unlike Outlook, you don’t need to install an add-on to be able to turn an email into a task in Thunderbird. Task manager, as well as calendar, are the built-in features of Thunderbird.
To turn an email into a task in Thunderbird, right-click the email you want to turn it into a task and select Convert To -> Task.

A pop-up will appear. Add the pieces of nesessary information such as task title, start date, due date, and status. You can also add a reminder to make sure you are not going to miss the task once the start date comes. You can also set the description of your task on the Description field on the bottom side of the pop-up window. To attach a document, you can click Attach on the toolbar. Once done adding the necessary information, click Save and Close (Ctrl+L).

To view the list of the tasks you have added in Thunderbird, you click the Events and Tasks menu and select Tasks (Ctrl+Shift+D).

You can use the navigation tool (Show) on the left panel to filter your tasks. You can display the tasks based on status dan day. You can also find the tasks on a certain date using the calendar picker on the left panel.