Please enter any project and you will see small gear icons on the right side of any task in this project. Use the one at the very top (in the header) that says "Task actions" when you hover over it, click on it and choose "Email tasks to this project". You will see a new window with detailed instructions on how this feature works.
If you would prefer just to link emails as tasks, then you'll need to use one of our browser plugins (or the Outlook or Thunderbird plugin) and then you will see a "Add email as task" link at the bottom of your task list in any project.
The difference between these features is that the first one allows you to forward a message to a unique address assigned to your project and would create a task out of it making the subject line the task name and the mail content - a task note while the second option would only link the message from your Gmail, Outlook or Thunderbird inbox and would require opening it through the plugin as the message would stay in your inbox and would need to be opened there.