Using markdown, here is what would match your plaintext email version:
Knock knock.
Who's there?
Little old lady.
Little old lady who?
I didn't know you could yodel.
The five lines have been indented by 4 spaces.
This caused markdown to treat it as __code__.
Or, prefixed and followed by lines with 3 backticks:
```
Knock knock.
Who's there?
Little old lady.
Little old lady who?
I didn't know you could yodel.
```