I see that there is a header MSWorkflowTask, is there any way to set that in an email that is sent via sharepoint designer? I would like to send a customized email that outlook recognizes as a task.
If it is a regular notification email, probably not, at least not from within SharePoint Designer. You would also have to create and populate several other headers such as
But if you are just interested in emailing tasks to users and if your workflow is creating a task, you can customize the email with a link and ultimately manage it in Outloook. This page describes customizing those notification emails with Designer: http://office.microsoft.com/en-us/sharepoint-designer-help/send-e-mail-in-a-workflow-HA010239042.aspx
The sections you'll want to pay attention to are Include a hyperlink to the current item and Send your own custom task notifications.
A user can respond to those task emails like so: http://office.microsoft.com/en-us/outlook-help/respond-to-a-task-from-a-sharepoint-site-HA010105444.aspx