Vladimir Bach, issues with text formatting in Mailbox has already been reported and our developers are already checking into this. If fix will be developed it will be released in one of the upcoming builds.
If you would like to fix this now, good place to start with is editing : ow_plugins/mailbox/bol/conversation_service.php:
Code that needs editing:
$message->text = preg_replace("/\n/", "", $message->text);
$message->text = preg_replace("/<br \/>/", "\n", $message->text);
$message->text = strip_tags($message->text);
Note that you need to know preg_repalce rules to add needed condition. I recommend waiting for the update with the fix from us.