Mail lines

My wife and I both use Imacs.

I notice that if we send an email back and forth, when the email is returned to me, there are these vertical lines that are places alongside the left hand side.

Example

I send the following text

Hey How are you?

The reply come back like this:

Fine

| Hey How are you?|

|

|

|

|

In other words, the previous email text is shown, but verticle lines appear beside it.

This is very annoying because if I want to cut and paste the text, the verticle line sget copied with it, and I have to edit each one out.

I hoping there is a setting that can prebvent the verticle line from appearing. Is there a way to prevent it from appearing?

Thank-you for your time and expertise!