Filter for Emails Added as Tasks
Since Todoist already indicates which tasks were added by email (or the Gmail plugin, at least) with a small envelope icon, it would be a great addition to be able to filter tasks that are added in this way.
Since most people probably have multiple projects that emails would go into, this filter would enable users to look at all their email-related tasks at once without having to tag each of them individually. This would be especially useful for users with a context-based workflow, since sometimes it is nice to see all the emails that still need a response in a single list.
This would avoid the necessity of tagging each of these tasks, or placing all emails in a single project, and assist in encouraging users to get out of their email inboxes when doing work, and into Todoist.