This is an interesting question! As it stands, you are already able to add Tasks via email to either:
A) Your Asana “My Task” list or
B) A Project
The most important part of this process is that you have registered the email you’re using to create Tasks from, in the “From Email” section of your “My Profile Settings” window.
Once your email is registered, if you send an email to either email@example.com (to create in your My Tasks) or x+projectID@mail.asana.com (to create the Task in a Project), the system will recognize the email and convert it regardless if it directly to tat email, or if it was CC-ed or BCC-ed.
Since the system does not recognize the client you are writing to, you will get an automatic reply from our system saying:
"Your task was successfully created in Asana, but we’re sending you this email because someone you listed in the To: or CC: field in your email is not a member of the Organization, Workspace, or project to which you’re sending.
You can learn more about this in the Asana Guide: http://asa.na/p6vin"
Receiving this email means the the system has created the Task from your email! The caveat here is that you because your client is not registered in Asana, you cannot assign or apply a due date to the Task via the email itself. You’ll need to do so only after you have logged into your Asana space.
Hope this provides some clarity on what is possible, though assigning a due date via email is still a worthy idea to add and vote on in the product feedback category here.
Let me know if you have any questions Valerio!