Extra line-breaks when posting by e-mail

I’m having this problem in WordPress 2.5, too.

Here’s how I fixed the line breaks:

I used the Text Control plugin:
https://dev.wp-plugins.org/wiki/TextControl
Install the plugin package into WordPress.

You will now find text control config in your WordPress SETTINGS

Within text control config, change the default post setting to MARKDOWN

If you have PHP v. 5x, you will need to replace the markdown.php file with an updated version. Otherwise, you will get errors on every post about Warning: preg_replace_callback()

Here is the updated version of markdown.php:
https://michelf.com/projects/php-markdown/