This gets asked a lot currently. The sentiment seems to be that email is doomed from a security viewpoint. You'll be leaking metadata no matter what. Even if your email provider was somehow secure, you have no way of knowing if the person (and their provider) you are communicating with is secure. If it's not (which is very likely) then all your efforts to secure your end are almost for nothing.
So yeah, my suggestion currently is to acknowledge the level of secrecy (or rather lack of) available with your email setup and use it accordingly.
Modern post companies use OCR and other digital processing to handle all mail, including postcards. At least is would be easier to mask who is sending the postcard, for example by omitting the sender details and throwing it into a mailbox far(ish) away from your home. Not a perfect system, as one could probably locate your home via triangulation if you were to use a number of different post boxes in the area of your home.
So yeah, my suggestion currently is to acknowledge the level of secrecy (or rather lack of) available with your email setup and use it accordingly.