[Adoby]

You have two basic options.

1. Calibre can treat blank lines as paragraph separators.

This is all in paragraph 1. This is all in paragraph 1. This is all in paragraph 1. This is all in paragraph 1. This is all in paragraph 1.(cr)(lf)
This is all in paragraph 1. This is all in paragraph 1. This is all in paragraph 1. (cr)(lf)
This is all in paragraph 1.(cr)(lf)
(cr)(lf)
But this is paragraph 2. But this is paragraph 2. But this is paragraph 2. But this is paragraph 2. But this is paragraph 2. But this is paragraph 2. But this is paragraph 2. But this is paragraph 2. (cr)(lf)
(cr)(lf)
And this is paragraph 3. (cr)(lf)
And this is paragraph 3. (cr)(lf)
And this is paragraph 3. (cr)(lf)

will become:

This is all in paragraph 1. This is all in paragraph 1. This is all in paragraph 1. This is all in paragraph 1. This is all in paragraph 1. This is all in paragraph 1. This is all in paragraph 1. This is all in paragraph 1. This is all in paragraph 1.

But this is paragraph 2. But this is paragraph 2. But this is paragraph 2. But this is paragraph 2. But this is paragraph 2. But this is paragraph 2. But this is paragraph 2. But this is paragraph 2.

And this is paragraph 3. And this is paragraph 3. And this is paragraph 3.

2. Or you can let Calibre use every (cr)(lf) as a paragraph ending.

This is all in paragraph 1. This is all in paragraph 1. This is all in paragraph 1. This is all in paragraph 1. This is all in paragraph 1. (cr)(lf)
This is all in paragraph 2. This is all in paragraph 2. This is all in paragraph 2. (cr)(lf)
This is all in paragraph 3. (cr)(lf)
(cr)(lf)
But this is paragraph 4. But this is paragraph 4. But this is paragraph 4. But this is paragraph 4. But this is paragraph 4. But this is paragraph 4. But this is paragraph 4. But this is paragraph 4. (cr)(lf)
(cr)(lf)
And this is paragraph 5. (cr)(lf)
And this is paragraph 6. (cr)(lf)
And this is paragraph 7. (cr)(lf)

will become:

This is all in paragraph 1. This is all in paragraph 1. This is all in paragraph 1. This is all in paragraph 1. This is all in paragraph 1.

This is all in paragraph 2. This is all in paragraph 2. This is all in paragraph 2.

This is all in paragraph 3.

But this is paragraph 4. But this is paragraph 4. But this is paragraph 4. But this is paragraph 4. But this is paragraph 4. But this is paragraph 4. But this is paragraph 4. But this is paragraph 4.

And this is paragraph 5.

And this is paragraph 6.

And this is paragraph 7.

You decide which in Preferences | Conversion | TXT input. Toggle "Treat each line as a paragraph".

I suggest that you use the first option. Don't treat each line as a paragraph. Use your text editor as a wordprocessor. Insert a blank line where you want a paragraph break. Let the editor and Calibre handle the linewrapping.

For this to work as you like your input should look like this:

title(cr)(lf)
(cr)(lf)
author(cr)(lf)
(cr)(lf)
A paragraph of text containing several sentences. A paragraph of text containing several sentences. A paragraph of text containing several sentences.(cr)(lf)
(cr)(lf)