"In your replies"? Please explain.

Thunderbird generally quotes the original message, with indentation to indicate multiple levels of quotation. It normally uses a '>' character, or coloured bars. I don't understand what you mean by "margin".

Below is an example of plain text with coloured bars indicating two levels of quotation.

This is what I mean by "left Margin" (see screen shot attached). There is no space (margin) on the left. The left edge in the screen shot is all the way out and there is no more beyond the edge.

I have a subject line before I get to the body of a message from the address. Are you using the conversations add-on by any chance?

No. I'm not. The only add-on I'm using is mail merge.

I have a few emails that can look like your image.

In all cases where I use 'reply' the small left indent margin has been lost, it is because of the css which was used by the original sender of email.

When you Reply, the original email including all html css is included and now becomes part of the 'Reply' email you are trying to create and send. The css coding is influencing all of the email and causing the issue.