On the iPad, you can have both the Mail app and the Reminders app open at the same time in split view and drag an email to the Reminders app. This will create a new reminder (aka task) and will attach the dragged email to it. If you use the Reminders on iPad or iPhone, taping on the newly created task will get you to that email in the Mail app. If you use another piece of software to handle your tasks (e.g. Exchange/Outlook), you cannot see the attached email, so you cannot know what the task is for. So, this method is clearly limited to usage in Apple iDevices.
On the iPhone, there is no slit view, so you cannot perform the above actions. However, you can use a number of apps instead. AirMail has the ability to send an email to Reminders and create a task out of it. The only downside is that it only adds an AirMail link to the newly created task. Taping on that link will get you to that email in the AirMail app. So, again if you use another piece of software to handle your tasks, you cannot know what the task is about. Also, the Preside app is an excellent choice for sending an email to Apple Reminders and, thus, create a task. The good thing in this case is that it adds both the email text and a Preside link to the task, so you can figure out the purpose of the task and handle it in both iPhone/iPad and other pieces of software. It also adds the email's subject as the task title. Preside also offers a great host of functionality (e.g. Exchange folders, categories, multiple configuration options etc), so it's worth trying out.
I hope that this helps.
