As part of the API changes made in the new Windows Phone 8.1 XAML programming model, the EmailComposeTask - and several other tasks - that were available in the Microsoft.Phone.Tasks namespace in Windows Phone 8 are no longer available.
Most of these have been replaced with a new way of performing the same tasks. For sending an email from within your app, the old way in Windows Phone 8 was as follows:
EmailComposeTask emailComposeTask = new EmailComposeTask();
emailComposeTask.Subject = "Subject";
emailComposeTask.To = "email@example.com";
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