I have the same problem. I know that forms can be fixed with one of the following solutions (both worked for me):
define('WPCF7_AUTOP', false);
add_filter('wpcf7_autop_or_not', '__return_false');
But I couldn’t find a proper solution to avoid unwanted p and br tags in HTML emails. The only way seems to be to edit the file “contact-form-7/includes/mail.php” – as described above by @icodina.
Is there any better way to prevent Contact Form 7 from applying the wpcf7_autop()
function to emails?