How do I take advantage of the wallpaper (background paper) of an email I received for the preparation of future post office.

You can use any image as the background for your message template.

Go to Menu > Tools > Settings > Mail > Templates and Signatures, and create a new template.

Then choose the background image.

Ok, Gari.
However, I would like to take advantage of the background image of an email I received to use in future emails

Yes, once you have saved it, follow the instructions above.