Hello, I hope this helps. For native speaker it comes out naturally and they often don't have to be consciously thinking about it. I'm not a native English speaker so I can relate to the struggle of using prepositions. The way I get around it is by imagining pictures or scenes. I use 'on' if it could be on the surface of the noun (We can say: I put spare dollar on the back of my phone. Because the money is on the SURFACE and not INSIDE the phone) I use 'in' if it could be inside the noun (e.g. in the box) Now for the email imagine yourself putting letters (actual ABCD cutouts) inside a mail envelope and not just on the surface of the envelope.

This might sound crazy or cool depending on how you think about it. But I always find visualization helpful in my pursuit of learning language. Anyway enough about me, thank you dear reader for reading until the end :)

In the email. In the book, in the newspaper, in the magazine, in the letter, in the sentence, in the paragraph, in the writing….

On the screen, on the wall, etc.

Yes, it’s in not on.

In the email